If you break that up you end up with only a few large and likely advertisement funded instances being able to survive.
I'm not saying I don't think instances should be able to use that model, only that I think that model should not be the dominant way of building a community on the fediverse. But I don't see why a user would be less attached to a community just because its hosted on a different server from them, especially on the threadiverse which is topic based and where users are most likely going to engage in multiple topics.