There's an MC Frontalot song called "Secrets from the Future" and the refrain is "You can't hide secrets from the future." It's something of a useful mantra to remind oneself that if "the future" is a part of your threat model, yes your encryption likely isn't enough because on a long enough timescale it is likely "the future" will crack it.

As with any other security issue, the question is "what is your threat model?" You can still justify encrypting your backup today if your threat model includes today's actors, however much you worry about "the future".

> 10 years from now, it might be as good as plain text.

Or 10 years from now it might be the next Linear A tablets to confuse cryptoarcheologists, unreadable and untranslatable and entirely foreign. If "the future" is in your threat model, don't forget the other fun forms of entropy beyond encryption being cracked such as encodings changing, file formats falling out of service/compatibility, "common knowledge" about slang or memes lost to the ages leaving things indecipherable, and so on and so forth. Most of those things are probably unlikely on only a 10 year time horizon, but you never can tell with "the future".