Unfortunately, while this works, it does not align with the goals of the companies running these forums-- to get as big as possible, to get the biggest valuation and/or slice of advertising dollars as they can. Even small monetary barriers to entry decrease participation substantially, and that's just not acceptable.

This is why I'm quite pessimistic about the current situation: the current in-vogue business model of surveillance/advertising capitalism demands massive size beyond what can be moderated, and thus makes this problem inevitable. And it only gets worse when the most toxic users are the most profitable, viz. Twitter's refusal to ban Donald, even though by any reasonable interpretation of their TOS, he breaks it every other day.