You really don't need to, if clients implement the outbox model correctly, and unlike Bitcoin, where every node needs to keep a copy of every single transaction in order to maintain censorship resistance, consensus, and full auditability of the supply, that is not the case for Nostr. We ONLY have the censorship resistance piece to worry about, and not every relay can store every user's notes, so being selective about the relays you post to is both wise and courteous.

One of the best things about Nostr is that you can personally host your own content privately, too. This way, even if all 5(or however many you use) of the relays you currently post to decided to ban you, you can simply change your public relays to somewhere else and rebroadcast your notes from your private relay. The reality is, even this is very unlikely to happen unless you really did something deserving of the ban such that all 5 relay operators banned you in quick succession.