It's already perfectly feasible to do. Meta/Apple etc. can just deploy a client that decrypts the message, scans it, re-encrypts (with a different key) and send

It's already perfectly feasible to do. Meta/Apple etc. can just deploy a client that decrypts the message, scans it, re-encrypts (with a different key) and sends it to their storage where they can store it forever and decrypt if needed. This way they could even have different clients in different regions still being compatible. It's just that it would suck and would not be secure any more.