So, I am surprised the Fediverse is not mentioned. Already bigger and better than anything Nostr is trying to do. Why reinvent the wheel? Or why not join the wheel?
I don't think this is always true. Centralized services have practical benefits, and federation is a compromise between fully decentralized and centralized.
Federation frees individual users from key management (a trivial issue for HN users, but a big pain for less technical users).
Instance admins can handle spam and abuse for entire communities, and what is acceptable varies between communities.
Data can be aggregated per instance instead of independently fetched per user. For example, previews for external links can be cached per instance, since you trust your instance. In a fully distributed system it'd be either vulnerable to manipulation (since HTTPS doesn't offer a way for others to cryptographically verify data originates from a URL, without contacting the origin server) or every user would have to fetch a link preview themselves, which has DDoS potential and tracking risks.
