And the mere fact it's a default makes it a significant problem when discussing it as a popular and widespread E2E messenger. It would be more borderline if it was a required configuration choice with no default that clearly disclaimed the ramifications.
Even then, you have the issue that you are not the only person with a copy of the conversation. Your partner - or partners - has it too. Does Apple require some kind of pre-conversation negotiation to determine how the conversation will be stored in the backups? Or at least provide some kind of warning if a person with backups disabled gets in contact with somebody with backups enabled?
I don't disagree with you about backups, but how useful they are is completely irrelevant in this context for several separate reasons.