Sure, but you already have no way of knowing which one the other parties in your chats are doing.
I already sync my Signal backups to the cloud, because that's the most practical and time/cost-effective way to have a 3-2-1 backup system for my chats.
There's a difference between someone in your chats acting adversarially and Signal supporting/encouraging adversarial behavior as part of the way the app works. If Signal published a change to the protocol that removed forward secrecy, we wouldn't consider it a non-event and say "well anyone could screenshot messages anyway," even though that may be true. They're calling this "secure backups," but in truth it appears to reduce security
I don't think it's appropriate to call someone you're talking to with disappearing messages turned off making a backup of the conversation so they have the (non-disappearing) message history if they drop their phone in a lake as "adversarial behavior".
If you don't want them to have a history only communicate via disappearing messages.
I'd also wonder where this shared encryption key for message "backups" is stored. If it's available on all of my devices, I suspect it would be available on other devices as well?
