you are mistaken, Nostr isn't better than Mastodon because it offers unique storage solution. You can backup your Mastodon posts just as well, the difference is Mastodon server owns your identifier, so even if you upload all your posts to your own Mastodon server, you lose your identity (and thus followers). Nostr commits posts to your public key, and demands that your followers look for you using that key. of course, it doesn't answer how should your followers find you except by a coincidence of publishing and reading from the same relays. Pkarr.org offers an answer to that question, but it is not compatible with Nostr keys unfortunately
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. :)