There was a mouse Utopia experiment, Universe 25, conducted by John B. Calhoun. Rats were allowed to live in a predator-free environment with all the resources they needed. As the rat population increased, they began to show social pathologies. Their environment was meant to hold more than 3,000 mice, yet the population peaked a bit over 2,000 as the mice started to exhibit social pathologies. Eventually, the population collapsed and was headed to extinction. Another experiment was conducted with Norwegian rats with similar results.

People are possibly more resilient than rats, one would hope. But there may be some social consequences to overcrowding that are inherent to large societies.

As you point out, we have developed coping mechanisms such as mental health therapy. I would argue that religion also falls into that category of coping mechanisms. It's hard to be dysfunctional or sin out in the desert alone. These things matter most in a social setting where unacceptable behavior affects others and then rebounds to the affected individual.

Human population has not yet peaked. However, it has been slowing down and is expected to eventually decline in a few decades. There is no telling if humans are mirroring the Universe 25 experiment. But if we were, we could expect an escalation of dysfunctional behaviors over the coming years.

Something that would mitigate such a doomsday scenario is that humans have ways of removing dysfunctional people from society, but this usually only occurs after they have caused harm to others, which in turn causes their victims to suffer their own dysfunctions.