>people on the other side of conversations with me can't just export everything with a button press.
Once any data is off your device and (decrypted) on someone else's, you must assume that they have full control over it, which includes backups. Anything else is poor privacy practice, security through obscurity.
In principle you're correct, but in practice, the lack of an export feature is enough to make sure that 99 out of 100 conversations don't get leaked to third parties. You don't know with perfect certainty which ones do get leaked anyway, but...
99 out of 100 conversations wouldn't be leaked to third parties even if there was an encrypted backup feature - which there is, is android. You're trading hostile inconvenience for basically zero benefit.
Once any data is off your device and (decrypted) on someone else's, you must assume that they have full control over it, which includes backups. Anything else is poor privacy practice, security through obscurity.