Every platform is nice at the start, for mainly the reason you say: people who join early have an interest in the platform, so we actually try to keep it nice.

Then every successful new platform gets its own eternal september. A large influx of people who don’t care about the platform at all, they just want to use it to talk to people. And of these, yes many are still nice people, but also many aren’t.

You see this in all kinds of communities, not just online. If you’re in a new or niche hobby, everybody there will have an interest in improving the hobby and the small community, so it will probably be very nice. When it gets mainstream (I’m looking at MTG here, but other people probably know other examples) then it starts to attract people who do nothing but complain.