I also see it as a good thing, why? Because you have instance moderators, and community moderators. If we join all communities together like that, were going to be essentially forcing these disparate community moderators to work together. This will never work, as people have different ideas of what should and shouldn't be allowed, and how to handle different issues (or even wether something is an issue in the first place)

Also there's the idea that if you don't like one community in one instance, you can move or create one in another instance, and everyone is happy - you could say that in this case redundancy is good, in case one falls into shit.

Is it a problem that people with a common interest, cannot find common ground ? Yes, it is a fatal flaw for a social network.

Sure, and client viewer can activate a "local posts only". It's easy to exclude everything else.

But you can't undelete what never existed. If there is no common space there is no community.

The best compromise Lemmy has is "the one big community" that drowns out everything and sucks the air out.

I think so, especially as users can't have multiple subscription lists. The post sorting algorithms use activity as a key metric, so lots of fragmented communities will get buried under posts from (for example) memes@lemmysworld.