Relays probably won’t keep your notes forever. The point is that everyone has the power to download their content and send it to a new relay. Better backup/archival solutions will emerge.

Very true. If you want to make sure your older notes don't disappear you need to take it into your own hands. Solutions like Nostr relay tray for desktop and citrine for Android are good local backup options. Haven relay is a good option if you have access to PC hardware or a VPS, and feel comfortable with configuring and hosting your own solution. If that is beyond your capabilities relay.tools has a good hosted option to run personal relays. I'm sure I'm the future this will only get easier.