I'd imagine that the complaint is that they're old and don't particularly meet UX expectations in the 20s.

I do like them, and have great memories of them, they were my childhood. But I do think we're overdue for a modern forum stack. Discourse was a step in that direction, but a lot of people strongly dislike it, and it doesn't seem to have gotten the reach of others before it.

A real contender for "new forum software" would have to address the fact that Discord is its competitor, not other forum software, and it would have to have a very compelling answer for "why this instead of Discord".