Most people do not like casually being exposed to such content, and so any sort of soft policy against it results in users self-selecting out of the social ecosystem. It's no longer just your decision, but one that affects the entire platform. And soon the entire platform becomes dedicated to that disturbing content because everyone else has left.
You could go to Parler or Gab or whatever site right now to see the results of your experiment in action and see why user self-moderation leads to the destruction of the site. This is why users offload that mental stress to the owners of the site who then hire people to manually filter the cruft themselves (often for the worse of the people doing said filtering, but that's a whole 'nother thing).