As much as i like Apple-i have some ios mobile devices and these things just work-but whether the backup is encrypted or not is not a diffrence. Most of the data about the user they get whenever he uses the devices. If they only collect that and combine it, it is easy to tell what will probably be in your backups.
Also with their scanning on device approach is a hint to that. That is not Apple alone, Google does the same with their integration of live text for audio, foto scanning for text and faces, and so on. It looks convenient in first place, just like the ios ecosystem, but in the end it is all about collection of data.
What does it help to encrypt your contacts for example. I can do that, but about 100% of the people i call do not. So at their end, all my encryption would be useless. Same goes for pictures. You can hide yours from the companies, but your friends and colleagues will take a picture of you once in a while and then it ends in the big scanning wheel.
I have no doubts that the intelligence services have their foot in this and sooner or later they will have personal data about anybody in real time.
The only solution then is, that anybody either does not use the internet for personal information (might never happen), or people shift to degoogled and deappled devices like Graphene and others (giving up a lot of nanny software that keeps them hooked like a slot machine), or anybody will live in a totalitarian world.
This is not something the individual can prevent or run away from. Depending on what the masses will do, in the end, just abandon everything digital might make us free again.