You can add more relays and you can rebroadcast any notes yourself to any number of relays, bitcoin full nodes keep all txs history, nostr relays not necessarily, but the concept is similar, a relay can go out, damus already nuked everything once, but your notes remain in other relays. So to the question you asked, it's diferent bc no single entity can erase or own your data, you have freedom to spread and broadcast as you wish. Another question one can ask is, what nostr data do you want to keep forever, is it worthy to keep social media notes forever? In my personal opinion no, some other stuff yes probably.