Thing is, I don't think most people want to replace subreddits, Facebook groups or HN. The issue is that for two of these three communities, they are hosted by a company that has a large amount of power over the communities. It doesn't make sense to replace HN with some federated/p2p alternative, because HN is already independent.

And not everything needs to be a decentralised system, that's just a false dichotomy. If GitLab.com deletes my repos or stops offering the features I like, I'll just move elsewhere, I could even move to another GitLab instance. It's much harder to move a group of people.

Besides, not everyone needs to run a server in a federated environment. As long as it's a possibility, one rogue actor can't take over the whole network. Most people don't host their own email/Matrix server, and that's okay.