As has been demonstrated time and time again, especially on the Internet, unmoderated discussion boards do not scale. Trolls can naturally push out the reasonable people by increasing the noise level. Once the number of users exceeds some small threshold it is basically a guarantee that trolls will move in. Shitposting is cheap, easy, and the people who do it have all the time in the world. If you don't moderate the board will become useless for substantive discussion.
I mean this was amply demonstrated back in the Usenet era. Nothing has fundamentally change with human psyche since then, so the rule still holds true. Twitter/X is just the lastest example.
You've hit the nail on the head here. If you let the trolls in they will suck all of the air out of the room.