I don't think we've consolidated around fediverse@lemmy.world. You're using a single post as an example. I've posted links that got 40+ comments in fediverse@kbin.social but way less in other communities. I've posted or seen threads in fediverse@lemmy.ml that got more discussion.
The merge of cooking communities on lemmy.world is also not really relevant. Those communities were each supposed to be specialized communities, not general cooking communities. They shutdown because they couldn't sustain enough activity. And they were all on lemmy.world so the userbase likely all overlapped; I'd bet that most ppl subbed to them were already subbed to cooking@lemmy.world anyway.
What I'm talking about is when small and medium sized servers (not lemmy.world) have their own communities that overlap with other communities. Users who join those servers aren't necessarily going to know about lemmy.ml or lemmy.world. They'll see communities they're interested in and sub, but then won't see as much interaction as they want. This leads to ppl just giving up and going back to the corporate sites.
Even if consolidation is happening, there's a transition period where ppl are posting in multiple places, ppl get the same post in their feed multiple times, comment threads are separate. Then when consolidation happens, you have a single community where those mods hold all the power. If we used something like the proposal above, each community could still exist but all the conversations are still consolidated. That keeps the power spread out and likely keeps each mod team in check and provides multiple on-ramps to the community. You could find movies@a.com or movies@b.com but if they're grouped, you still find the super-community. And then if one of those servers goes down, only users subbed to that community have to migrate and they should be tangentially aware of the other community so migration is easier. Their server could even handle that migration automatically.