> The root problem is that attention has gone from abundant to scarce

I don't think that's necessarily true.

I think the root problem is that running & using a forum is too difficult. That is why centralized forums (like you mentioned, reddit and facebook) that handle it for you won out against decentralized forums run by forum members.

Even before facebook/reddit/etc forums tended to live or die by individual effort of one passionate system admin dealing with all the hosting, updates, accounts, and spam until they get fed up and the forum closes because they can't find someone else to take the keys.