The thing is that, technically, "human fluorishing" (understood as the evolutionary tendency of our specie to thrive & expand) is not something that can be maintained indefinitelly.
I meant "human flourishing" as a shorthand for the list of things I listed, as in "things that tend to make individual humans feel fulfilled" not the expansion and thriving of humans as a species. I don't think the latter is always seen as utopian; for example if I were to list utopias like The Culture, The Federation in Star Trek, Le Guin's short stories, the Abbey of Thélème... Some of those do feature human expansion although even there it's not uncomplicated (The Federation not only explores but also colonizes uninhabited worlds and I think it's fair to see "the expansion of the human species" as part of its utopian vision; I think the same is true of The Culture but the books also challenge the idea), others straight-up reject it like many of Le Guin's utopias, and I think ancient versions of the genre like the Abbey of Thélème don't think that much about it at all. However all of those utopias portray humans as having or being able to achieve a variety of "personal fulfillment" goals such as those I listed; those are what I meant. I do think our evolutionary tendency to thrive & expand may be worth valuing for its own sake, contra Le Guin, but that's a different conversation.
Having said that I don't think the "rat utopia" experiments say that much about human flourishing. For one thing those "utopias" didn't meet all of the rats' needs - they had unlimited food and safety from outside threats but they didn't have unlimited space or the kind of stimulation they evolved to thrive and maintain their social structures in. I guess it's good nuance to understand that "flourishing" doesn't reduce to "unlimited food and safety from predators" but that organisms have other needs too (notably space), but I think it's something most people realize already. Note that stories that do feature "the evolutionary tendency of our species to thrive & expand" as utopian tend to have the opposite of a "rat utopia", with space colonization/exploration making space unlimited but with challenging conditions.
I'm also not convinced such behavioral sinks apply to humans, or at least apply to them as completely as they did to those rats. Some unique features we have that seem relevant here include our level of sociality, playfulness and adaptability. Humans are much more social than our closest relatives (& maybe all mammals) so overpopulation doesn't have the same impacts on us as others. We also (literally) play a lot more than any other species, in the sense of engaging in behaviors for the sake of random goals instead of the more straightforward ones that usually motivate us - in that category I'd list not only what we understand as play and games, but also things like art, science, sports, random hobbies, etc. We don't only individually play, but as cultures we devote time and resources to goals "for their own sake" instead of concrete survival/expansion. I'd guess such random behavior serves as a natural outlet in cases where conditions are "too favorable", one that we probably literally evolved to engage in (evolutionarily speaking play has the purpose of learning new things, and you do it when conditions are favorable enough that you don't need to focus on survival), meaning it's likely to feel satisfying to some extent at least. Finally, Alison Gopnik says adaptability is a hallmark of humans as a species and I think that claim holds up - human societies have proven able to adapt to a huge variety of environments, both physical and social. Our own societies are extremely different from the kind we evolved with and have tons of issues, but they still basically function, with a huge proportion of humans in them leading lives that range from satisfactory to fulfilling, in a way that wouldn't be true of a comparable number of chimpanzees. So I have doubts that we'd completely collapse as a species because of something so generic as conditions being "too favorable". Humans and human societies can be broken, no doubt about that, but that usually involves extreme scenarios. Unfavorable ones, at that.
It might be worth noting at this point that a lot of us, particularly of the "posting randomly on the internet" variety, do functionally live in "rat utopias" with unlimited food, no predators but limited space and tons of people around. And I think most would attest that while it's not the key to perfect happiness, it also hasn't devolved into the horrifying hellscape the rats experienced.
This isn't to say I think human utopia is possible/coherent/compatible with our nature. I just don't think the rat experiment is a very good example for that argument.
This question is so generic I can't help but feel there is a more specific idea behind it. Can you talk about what made you want to ask this, what kind of answers you're expecting?