There should be no expectations of data being saved anywhere if the user is not paying to keep it there. In the end the data will always be deleted, in months or years, but it will ALWAYS be deleted.
Eventually, uploaded multimedia will be the heaviest load on all free and open social networks (ActivityPub, ATproto, Nostr). Holos Social, a new "relay on the phone talking to a remote relay" Mastodon client has the option to store the user's own multimedia data on a webDAV or an S3 drive. I'd say that's a pretty good option to ensure personal data ownership while the network itself is functional, although I still have to try it to see how it works exactly, and how it displays for other ActivityPub users. I'd love to see something similar in Nostr clients.