The primary issue in my mind is that the api-s don't follow one well-defined standard, like signed Nostr events queried with filters.
This way you don't have to keep rewriting interfaces, and all data is verfiable.
The "closed" part is tricky: Even with wide-spread Nostr adoption I don't think that we would have completely free access to data.
Data is precious and people will protect it with digital fences, even if everything is signed and standardized.
But to the degree everything is interoperable and verifiable, we are in a very good place.
Even small backups can help in a major way. Real censorship becomes too costly in practice even for a large player.
It will result in more competition and therefore more openness as well.
