There are many options to account for this:

Can use Blastr to hit basically every relay of any size for free.

You can host your own or backup your content, which is very easy with Start9 or Umbrel. If somehow every relay nuked you then you lose nothing and can search for new relays that accept your notes.

Nuked relays often have little or no real effect. Will nuked the biggest Nostr relay (Damus) and zero notes were lost.

The upcoming outbox model also solves most of these concerns and is being adopted by basically every Nostr client.

Summary: you are never “tied” to just one relay or client. That’s part of the beautiful of Nostr being an open protocol.