the major issue with forums, as pointed out, is the hassle of having to go from one website to another to talk about various subjects and needing to sign up to each one of them.

Honestly the "having to sign up" part would be trivial to solve if topical forums just globally adopted OpenID sign-in or similar. No need to have one account per community if you already have (or "are") an account in the World.

But even then, there's a point to having to go through a sign-up process. At least some sort of vetting. We have seen how far have fallen all the communities that have ever relaxed sign-ups (as another comment in this thread shows, there was once a time when FB only allowed educated people in).