Surely in the context of "End to End backups" the user is the "End" on both ends. The servers shouldn't have the keys. They can be as compromised as they want,

Surely in the context of "End to End backups" the user is the "End" on both ends. The servers shouldn't have the keys. They can be as compromised as they want, but the most they should be able to see is when, how much and where from you're backing up data, but not the actual data.