We already have sites that offer small communities; what we needed was a replacement for Reddit.
The thing is neither Lemmy not kbin were made to be Reddit replacements.
The reason Reddit/Facebook/et al are so huge is because people want to have a single community to talk with
They're also centralised and their respective servers can tolerate much more content being shared/posted onto them than a fediverse server can. Having different communities allows for a given server not to be saturated.
Idk why there are so many general use instances when the threadiverse would be a great place for themed instances to exist. But even with thematic instances you can't avoid similar communities existing (because multiples servers could exist for the same topic/fandom/etc.
Something that connects communities about the same topic from different servers into a "macro-community", so everything posted in any of the communities can be read when you click one of them, without putting to much pressure into a sole server, would be cool. But I don't think is posible with AP alone.