Personally, I think it's less that centralization is a law of the universe and rather, things are cyclical. Atoms smash together to form molecules and amino acids and living organisms, and those organisms also break down eventually into smaller things again.

Likewise, with social networks and social hubs online, we've seen the cycle of centralization and decentralization of social networks, from disparate forums to web 2.0 social networks like Facebook, Twitter, and Reddit, and now an attempt to decentralize social networks once again. Maybe it will last? Maybe it won't? But at least we have this space right now, at this moment in time. (Also even subreddits decentralize, e.g. subreddit about topic "x" and subreddit about same topic but named "true x")

Or maybe all of this is just my late night ramblings lmao

Well, I guess email is one counterexample. Though we can all see its issues (spam, overzealous spam filters, complete lack of "feature development", even though that's probably a good thing there).

Also, another issue I'm worried about is horizontal scalability. I hope that as communities grow it won't become cost-prohibitive to run a new instance (as it will have to mirror too much content).