This is already possible if you self host your stuff. I’m talking about iCloud specifically here - backups are not encrypted so it’s either scan in the backend or scan on the client.

If you don’t want to be scanned you can turn it off. I honestly don’t see the issue. It seems the only thing people can say are hypothetical situations here.

Realistically, because if they don't have something to scan for it, countries / EU are going to require them to provide a backdoor to scan it. There are already proposals in the EU to that affect.