Comment by SAI_Peregrinus

The static symmetric key is fundamentally different from an ephemeral asymmetric key. We've no indication that symmetric encryption is vulnerable to "store now, decrypt later" attacks when used with a sufficiently long key, which Signal has. Non-post-quantum asymmertic cryptography is vulnerable to "store now, decrypt later" attacks, which is why forward secrecy is needed.

The backups feature doesn't open up any new vulnerability that didn't inherently exist in sending messages to someone else you might not fully trust. One person in a group chat can also take pictures of their phone's screen & upload your messages to the public.