I agree, the revolution with nostr is "own your identity".
But this don't exclude what I'm saying: on nostr the natural first database become the client.

Imagine this: you are writing a post on mastodon. You send it but no connection; for a connection error the post is frozen in your client. This is mean that this post "never existed"; the post never reached the server where the ultimate database resides. 

Now see the exact situation composing a nostr note. One/some relays doesnt received the notes? It still exists in the ultimate database that is your client. A relay not having some notes is intended to be possible; could be also an intentional choice to not send a particular note to some particular relays.

There could be exceptions (running out of storage on device...) but the shift I see is natural: with nostr clients are the new ultimate uncontested databases of information and this could enable a lot of cool feature.
Relays could be backups of this data, multimedia-server-storage (all that isnt "notes", see blossom for example...); but assignin them responsibility on notes-data-retention is a big step in a wrong derection in my opinion. :)