Spam. That's why usenet failed and email is an unusable mess. Ignore spam and you'll end up the same. Web based forums dealt with spam but there is nothing inherently centralized about spam prevention.
I'd say there is. You could crowd source moderation, but that's susceptible to the same manipulation seen with bitcoin miners.
Culturally speaking, we seek out trustable sources. I'm not sure technology can surmount that.
Why should finding a trustable source be incompatible with using technology? It seems like precisely the sort of problem that technology can help with.
Of course, technology alone can't "solve" trust, but it can keep track of identities and claims and reputations, and even allow implementation of clever algorithms for discovering patterns and making recommendations.
Anyway, here is the plan for decentralized spam/abuse prevention that Matrix is proposing, which looks promising:
https://matrix.org/blog/2020/10/19/combating-abuse-in-matrix...
care to elaborate?