I think the parent means encrypt customer data with key specific to that customer. When you erase that customer key their data becomes irreversibly damaged.
I get that, but the problem is the way data is stored today it is stored on a single volume. That is many customers are stored on a single volume. When backed up there is normally one key per volume.
I guess the real issue is who will be responsible ensuring backups are stored in a way that different clients are isolated.
As somebody who makes backup software I know the burden will at some point be on my plate.
That being said, if people stored data differently, and did actually have a key per customer then the backup software won't matter, because like the parent and you said, just delete the key. But nothing really works like that today, and it will require a massive amount of software to be rewritten to handle this sort of stuff. So until then either you can't backup your data, or you make the backup provider figure it out.
