I am shocked, SHOCKED! That no one has mentioned the John Calhoun Mouse Utopia Experiment yet.
What happened was that Calhoun, a scientist, made a heaven on Earth for rats. It had everything, food, water, housing. But it was in an enclosed space, and once the rats had reached a certain population, they no longer had the space they needed to stake out territories or engage in normal social behavior. Some of the male rats turned on each other while others disengaged from society and behaved like rat
hikikomori. Female rats forgot how to raise children and within a short amount of time, the colony developed a condition called "behavioral sink" and collapsed. Every time the Mouse Utopia Experiment was done, it ended the same way - with every mouse dying. This concerned scientists who thought the same thing could happen to humans if politics and economic forces crammed them all into cities. However, there hasn't been a mass die-off of humans yet, and scientists are pretty sure that humans, being slightly smarter than rats, won't follow the rat path to destruction.
Mice only lacked ONE resource they needed to reproduce - the space to stake out their own territories. That fact eventually caused their collapse. People today have plenty of space to mill around (and most millenials can afford a flophouse apartment for themselves,) but to have kids and raise them well, humans need a lot of resources they just aren't getting anymore - a large private space of their very own, a job with a respectable salary (or two jobs, depending on where one lives,) enough money to pay someone to take care of the childrearing duties (or to allow one parent to stay at home with the kids,) and the societal status that comes (or at least used to come) with being a parent. It doesn't help that we live in a society where having kids is seen as a money sink/ drudge work by both men and women. Or as a way to suck away money and time that they could have spent on themselves and their cool, flashy hobbies.
Still, after watching video commentary after video commentary about the Mice Utopia, I really didn't think humans would stand for societies that treated them like mice in cubicles, or that people would willingly emulate the conditions of the mice utopia all by themselves, but then, I didn't figure on a little island nation called Japan...