I think the whole point is to keep them from merging. No centralization. That way you can up and down vote servers along with post or block certain servers altogether. Now they might be able to incorporate it some way in the app where you could group common instances and assign your own categories. That would work pretty well.

I think there is something in the works that makes them multicommunities per default.

Decentralization is kinda the entire point of lemmy and other fediverse apps. There is no company, like reddit, facebook, etc. who controlls lemmy. Anyone can run their own lemmy instance however they feel like and create whatever communities they want.