If you're considering the ability to rewire one's utility function, why simplify the function rather than build cognitive tools to help people better satisfy the function? What you've proposed is that an AI destroys human intelligence, then pursues some abstraction of what it thinks humans wanted.
Your suggestion is that an AI might assume that the best way to reach its goal of making humans happy (maximizing their utility) is to attain the ends of humans' functions faster and better than we could, and rewire us to be satisfied. There are two problems here that I see.
First, the means are an end. Much of what we value isn't the goal we claim as our objective, but the process of striving for the goal. So here you have an AI that doesn't really understand what humans want.
Second, most humans aren't interested in every avenue of exploration, creation and pleasure. Our interests are distinct. They also do change over time (or some limited set of parameters do anyhow). We don't always notice them change, and when they do, we like to track down what decisions they made that led them to their new preferences. People value the (usually illusory) notion that they control changes to their utility functions. The offer to be a "wirehead" is an action which intrinsically violates peoples' utility in the illusion of autonomy. This doesn't apply to everyone - hedonists can apply to your heaven. I suspect that few others would want it.
Also,
If you don't want to be "reduced" to an eternal state of bliss, that's tough luck... The FAI can simply modify your preferences so you want an eternally blissful state.
That is not friendly.
I think you have an idea that there is a "global human utility function" and that FAI is that which satisfies this. Humans have commonalities in their functions, but they are localized around the notion of self. Your "FAI" generalizes what most people want in some form except for experience and autonomy, but the other values it extracts are, in humans, not independent from those.
I can conceive of the following 3 main types of meaning we can pursue in life.
1. Exploring existing complexity: the natural complexity of the universe, or complexities that others created for us to explore.
2. Creating new complexity for others and ourselves to explore.
3. Hedonic pleasure: more or less direct stimulation of our pleasure centers, with wire-heading as the ultimate form.
What I'm observing in the various FAI debates is a tendency of people to shy away from wire-heading as something the FAI should do. This reluctance is generally not substantiated or clarified with anything other than "clearly, this isn't what we want". This is not, however, clear to me at all.
The utility we get from exploration and creation is an enjoyable mental process that comes with these activities. Once an FAI can rewire our brains at will, we do not need to perform actual exploration or creation to experience this enjoyment. Instead, the enjoyment we get from exploration and creation becomes just another form of pleasure that can be stimulated directly.
If you are a utilitarian, and you believe in shut-up-and-multiply, then the correct thing for the FAI to do is to use up all available resources so as to maximize the number of beings, and then induce a state of permanent and ultimate enjoyment in every one of them. This enjoyment could be of any type - it could be explorative or creative or hedonic enjoyment as we know it. The most energy efficient way to create any kind of enjoyment, however, is to stimulate the brain-equivalent directly. Therefore, the greatest utility will be achieved by wire-heading. Everything else falls short of that.
What I don't quite understand is why everyone thinks that this would be such a horrible outcome. As far as I can tell, these seem to be cached emotions that are suitable for our world, but not for the world of FAI. In our world, we truly do need to constantly explore and create, or else we will suffer the consequences of not mastering our environment. In a world where FAI exists, there is no longer a point, nor even a possibility, of mastering our environment. The FAI masters our environment for us, and there is no longer a reason to avoid hedonic pleasure. It is no longer a trap.
Since the FAI can sustain us in safety until the universe goes poof, there is no reason for everyone not to experience ultimate enjoyment in the meanwhile. In fact, I can hardly tell this apart from the concept of a Christian Heaven, which appears to be a place where Christians very much want to get.
If you don't want to be "reduced" to an eternal state of bliss, that's tough luck. The alternative would be for the FAI to create an environment for you to play in, consuming precious resources that could sustain more creatures in a permanently blissful state. But don't worry; you won't need to feel bad for long. The FAI can simply modify your preferences so you want an eternally blissful state.
Welcome to Heaven.