I propose a goal of perpetuating interesting information, rather than goals of maximizing "fun" or "complexity". In my opinion, such goal solves both problems of complex but bleak and desolate future and the fun maximizing drug haze or Matrix future. Of course, the rigorous technical definition of "interesting" must be developed. At least "interesting" assumes there is an appreciating agent and continuous development.
I think we should start with asking what is meant by "flourishing civilizations"? In the AI's view, a "flourishing civilization" may not necessarily mean "human civilization".
I generally agree with Stephen Fowler, specifically that "there is no evidence that alignment is a solvable problem."
But even if a solution can be found which provably works for up to N level AGI, what about N+1 level? A sustainable alignment is just not possible. Our only hope is that there may be some limits on N, for example N=10 requires more resources than the Universe can provide. But it is likely that our ability to prove the alignment will stop well before a significant limit.
I see the biggest problem not on the technical side of things, but on the social side. The existing power balance withing the population and the fact that it discourages cooperation is in my opinion a much bigger obstacle to alignment. Heck, it prevents alignment between human groups, let alone between humans and the future AGI. I don't see how increased intelligence of a small select group of humans can solve this problem. Well, maybe I am just not smart enough.