My first reply to you focused on point number 2. But in retrospect, I've realized that your point 1 has utterly horrifying implications.
I don't find utility monsters hugely counter-intuitive for total util. If you replace the standard utility monster with something like "extremely efficient simulation of many consciousnesses", then converting all our inefficient fleshy beings into a much greater number of (lets say) similarly happy other beings seems a good deal to me.
If you want a more grounded form of the utility monster thought experiment there's no need to invoke futuristic technology. You need only consider the many disutility monsters who actually exist right now, in real life. The world is full of handicapped people whose disabilities make their lives harder than normal, but still worth living. If I am not mistaken the ethical theory you are proposing would recommend killing all of them and replacing them by conceiving new healthy people.
Any ethical theory that suggests we ought to kill handicapped people (who are leading worthwhile lives) and replace them with healthy people has totally failed at being an ethical theory [1].
When people have severe health problems or disabilities we do not kill them and use the money we save to pay some young couple to conceive a replacement. We take care of them. True, we don't devote all the resources we possibly can to caring for them but we still try harder than one would expect when taking the total view.
Why do we do this? I'd suspect it would be because people's ethics are more in line with prioritarianism than utilitarianism. Helping the least well off is good even if they are bad at converting resources into utility. Helping the least well off might not have infinite value, there might be some sufficiently huge amount of regular utility that could override it. But it is really, really important.
And no, "not existing" does not count as a form of being "least well off." Human beings who exist have desires and feelings. They have future-directed preferences. If they die they cannot be replaced. Nonexistant people do not have these properties. If they ever do come into existence they will, so it makes sense to make sure the future will be a good place for future people. But since nonexistant people are replaceable it makes sense to sometimes not create them for the sake of those who already do exist.
Now, you might rightly point out that while people seem to favor "prior existance" or "person affecting" viewpoints like this, there is a certain point after which they stop. For instance, most people would find it bad if the human race went extinct in the future, even if its extinction benefited existing people. What that suggests to me is that having a decent amount of worthwhile lives in existence is an important value for people, but one that has diminishing returns relative to other values. Preventing the human race from going extinct is a good thing, but once you've assured a decent amount of people will exist other things become important.
I think that this establishes at least three different values in play, which have diminishing returns relative to each other:
Amount of utility (I am unsure whether that is total or average. It may be some combo of both).
Prioritarianism, (I am unsure as to whether that is traditional prioritarianism, or whether something like Amartya Sen's "capabilities approach" is better).
Number of worthwhile lives (I'm not sure whether having a large number of worthwhile lives is literally what is valuable, or if what is actually valuable is things like love, friendship, diversity, etc. that can only be achieved by having large amounts of worthwhile lives. I suspect the later).
This ethical system can explain why people are willing to not create new people in order to benefit existing people in situations where the population is large, but would not do so if it would cause the human race to go extinct. It can explain why people feel we should still share resources even in "utility monster" scenarios. And it explains why it is bad to kill handicapped people.
(1]. I know Peter Singer has gained some notoriety for suggesting it might be acceptable to kill handicapped infants. However, his argument was based on the idea that the infants had not matured enough to attain personhood yet, not on the idea that it's okay to kill a fully developed person and replace them with someone who might enjoy their life slightly more. Your ethical theory would endorse killing fully grown adult people with normal cognitive skills merely because they possess some physical disability or health problem.
Let's imagine a life extension drug has been discovered. One dose of this drug extends one's life by 49.99 years. This drug also has a mild cumulative effect, if it has been given to someone who has been dosed with it before it will extend their life by 50 years.
Under these constraints the most efficient way to maximize the amount of life extension this drug can produce is to give every dose to one individual. If there was one dose available for all seven-billion people alive on Earth then giving every person one dose would result in a total of 349,930,000,000 years of life gained. If one person was given all the doses a total of 349,999,999,999.99 years of life would be gained. Sharing the life extension drug equally would result in a net loss of almost 70 million years of life. If you're concerned about people's reaction to this policy then we could make it a big lottery, where every person on Earth gets a chance to gamble their dose for a chance at all of them.
Now, one could make certain moral arguments in favor of sharing the drug. I'll get to those later. However, it seems to me that gambling your dose for a chance at all of them isn't rational from a purely self-interested point of view either. You will not win the lottery. Your chances of winning this particular lottery are almost 7,000 times worse than your chances of winning the powerball jackpot. If someone gave me a dose of the drug, and then offered me a chance to gamble in this lottery, I'd accuse them of Pascal's mugging.
Here's an even scarier thought experiment. Imagine we invent the technology for whole brain emulation. Let "x" equal the amount of resources it takes to sustain a WBE through 100 years of life. Let's imagine that with this particular type of technology, it costs 10x to convert a human into a WBE and it costs 100x to sustain a biological human through the course of their natural life. Let's have the cost of making multiple copies of a WBE once they have been converted be close to 0.
Again, under these constraints it seems like the most effective way to maximize the amount of life extension done is to convert one person into a WBE, then kill everyone else and use the resources that were sustaining them to make more WBEs, or extend the life of more WBEs. Again, if we are concerned about people's reaction to this policy we could make it a lottery. And again, if I was given a chance to play in this lottery I would turn it down and consider it a form of Pascal's mugging.
I'm sure that most readers, like myself, would find these policies very objectionable. However, I have trouble finding objections to them from the perspective of classical utilitarianism. Indeed, most people have probably noticed that these scenarios are very similar to Nozick's "utility monster" thought experiment. I have made a list of possible objections to these scenarios that I have been considering:
1. First, let's deal with the unsatisfying practical objections. In the case of the drug example, it seems likely that a more efficient form of life extension will likely be developed in the future. In that case it would be better to give everyone the drug to sustain them until that time. However, this objection, like most practical ones, seems unsatisfying. It seems like there are strong moral objections to not sharing the drug.
Another pragmatic objection is that, in the case of the drug scenario, the lucky winner of the lottery might miss their friends and relatives who have died. And in the WBE scenario it seems like the lottery winner might get lonely being the only person on Earth. But again, this is unsatisfying. If the lottery winner were allowed to share their winnings with their immediate social circle, or if they were a sociopathic loner who cared nothing for others, it still seems bad that they end up killing everyone else on Earth.
2. One could use the classic utilitarian argument in favor of equality: diminishing marginal utility. However, I don't think this works. Humans don't seem to experience diminishing returns from lifespan in the same way they do from wealth. It's absurd to argue that a person who lives to the ripe old age of 60 generates less utility than two people who die at age 30 (all other things being equal). The reason the DMI argument works when arguing for equality of wealth is that people are limited in their ability to get utility from their wealth, because there is only so much time in the day to spend enjoying it. Extended lifespan removes this restriction, making a longer-lived person essentially a utility monster.
3. My intuitions about the lottery could be mistaken. It seems to me that if I was offered the possibility of gambling my dose of life extension drug with just one other person, I still wouldn't do it. If I understand probabilities correctly, then gambling for a chance at living either 0 or 99.99 additional years is equivalent to having a certainty of an additional 49.995 years of life, which is better than the certainty of 49.99 years of life I'd have if I didn't make the gamble. But I still wouldn't do it, partly because I'd be afraid I'd lose and partly because I wouldn't want to kill the person I was gambling with.
So maybe my horror at these scenarios is driven by that same hesitancy. Maybe I just don't understand the probabilities right. But even if that is the case, even if it is rational for me to gamble my dose with just one other person, it doesn't seem like the gambling would scale. I will not win the "lifetime lottery."
4. Finally, we have those moral objections I mentioned earlier. Utilitarianism is a pretty awesome moral theory under most circumstances. However, when it is applied to scenarios involving population growth and scenarios where one individual is vastly better at converting resources into utility than their fellows, it tends to produce very scary results. If we accept the complexity of value thesis (and I think we should), this suggests that there are other moral values that are not salient in the "special case" of scenarios with no population growth or utility monsters, but become relevant in scenarios where there are.
For instance, it may be that prioritarianism is better than pure utilitarianism, and in this case sharing the life extension method might be best because of the benefits it accords the least off. Or it may be (in the case of the WBE example) that having a large number of unique, worthwhile lives in the world is valuable because it produces experiences like love, friendship, and diversity.
My tentative guess at the moment is that there probably are some other moral values that make the scenarios I described morally suboptimal, even though they seem to make sense from a utilitarian perspective. However, I'm interested in what other people think. Maybe I'm missing something really obvious.
EDIT: To make it clear, when I refer to "amount of years added" I am assuming for simplicity's sake that all the years added are years that the person whose life is being extended wants to live and contain a large amount of positive experiences. I'm not saying that lifespan is exactly equivalent to utility. The problem I am trying to resolve is that it seems like the scenarios I've described seem to maximize the number of positive events it is possible for the people in the scenario to experience, even though they involve killing the majority of people involved. I'm not sure "positive experiences" is exactly equivalent to "utility" either, but it's likely a much closer match than lifespan.