>Forums are good in the way that they force everyone to mostly stay on a single topic of discussion.

I have the complete opposite experience. Forum on-topicness depends on the moderators and users, not the format. I've been in plenty of forums and IRC/Discords where every thread and channel devolved into general chat. I find it less likely in the ephemeral comment threads of HN and Reddit.

The criticism is valid even within rule-following on-topicness. Threads encourage splintered digressions and neglect group cohesion. There’s no back pressure for considering the “room” - every branching thread is both an invitation to participate and a side room expected to be ignored without protest if a passerby is uninterested. Even when following the rules of the format.