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.

There is no reason to lump forums and IRC/Discord together here. If anything, the latter is closer to places like here or Reddit, where the discussions are ephemeral regardless of topicality, whereas with forums a single topic can go on for years.