other people are going to disagree with you or me
Of course, that's a given.
These are questions that the discipline of population ethics deals with
So is this discipline basically about ethics of imposing particular choices on other people (aka the "population")? That makes it basically the ethics of power or ethics of the ruler(s).
You also call it "morality as altruism", but I think there is a great deal of difference between having power to impose your own perceptions of "better" ("it's for your own good") and not having such power, being limited to offering suggestions and accepting that some/most will be rejected.
"morality as cooperation/contract" view
What happens with this view if you accept that diversity will exist and at least some other people will NOT follow the same principles? Simple game theory analyses in a monoculture environment are easy to do, but have very little relationship to real life.
whether we should be morally glad about the world as it currently exists
That looks to me like a continuous (and probably multidimensional) value. All moralities operate in terms of "should" an none find the world as it is to be perfect. This means that all contemplate the gap between "is" and "should be" and this gap can be seen as great or as not that significant.
whether e.g. we should make more worlds that are exactly like ours
Ask me when you acquire the capability :-)
So is this discipline basically about ethics of imposing particular choices on other people (aka the "population")? That makes it basically the ethics of power or ethics of the ruler(s).
That's an interesting way to view it, but it seems accurate. Say God created the world, then contractualist ethics or ethics of cooperation didn't apply to him, but we'd get a sense of what his population ethical stance must have been.
No one ever gets asked whether they want to be born. This is one of the issues where there is no such thing as "not taking ...
I've started listening to the audiobook of Peter Singer's Ethics in the Real World, which is both highly recommended and very unsettling. The essays on non-human animals, for example, made me realize for the first time that it may well be possible that the net utility on Earth over all conscious creatures is massively negative.
Naturally, this led me to wonder whether, after all, efforts to eradicate all consciousness on Earth - human and non-human - may be ethically endorsable.This, in turn, reminded me of a recent post on LW asking whether the possibility of parallelized torture of future uploads justifies killing as many people as possible today.
I had responded to that post by mentioning that parallelizing euphoria was also possible, so this should cancel things out. This seemed at the time like a refutation, but I realized later I had made the error of equating the two, utility and disutility, as part of the same smooth continuum, like [-100, 100] ∈ R. There is no reason to believe the maximum disutility I can experience is equal in magnitude to the maximum utility I can experience. It may be that max disutility is far greater. I really don't know, and I don't think introspection is as useful in answering this question as it seems intuitively to be, but it seems quite plausible for this to be the case.
As these thoughts were emerging, Singer, as if hearing my concerns, quoted someone or other who claimed that the human condition is one of perpetual suffering, constantly seeking desires which, once fulfilled, are ephemeral and dissatisfying, and therefore it is a morally tragic outcome for any of us to have emerged into existence.
Of course these are shoddy arguments in support of Mass Planetary Biocide, even supposing the hypothesis that the Earth (universe?) has net negative utility is true. For one, we can engineer minds somewhere in a better neighborhood of mindspace, where utility is everywhere positive. Or maybe it's impossible even in theory to treat utility and disutility like real-valued functions of physical systems over time (though I'm betting it is). Or maybe the universe is canonically infinite, so even if 99% of conscious experiences in the universe have disutility, there are infinite quantities of both utility and disutility and so nothing we do matters, as Bostrom wrote about. (Although this is actually not an argument against MPB, just not one for it). And anyway, the state of net utility today is not nearly as important as the state of net utility could potentially be in the future. And perhaps utilitarianism is a naive and incorrect ethical framework.
Still, I had somehow always assumed implicitly that net utility of life on Earth was positive, so the realization that this need not be so is causing me significant disutility.