I disagree that the centralization was useful. The isolated communities were healthier and funner as a result of being smaller and disconnected from other communities—the isolation worked towards their benefit not their detriment. No one could go trawling through your posts in other communities and decide they don't like you after all because you had an identity there that could be completely different from your identity in other communities.

I think the centralization is actually a huge contributing factor to the decline in overall health of the internet. We didn't evolve in a way that's compatible with seeing everything that our friends say to any of their other friend groups—we're wired for compartmentalized interactions, not global state.