Or to put it another way, if it's important enough to retain when storage space is low, it's important enough to retain for when you have to recover from a backup.
If I stored something specifically for offline use, I want it to be available as long as my device is with me, even when I'm offline. (1) Some of it is important as you say, and they should be backed-up, but (2) some are important, but is capable of being downloaded again and shouldn't be backed up and shouldn't'. The latter is the one that is now automatically cleaned up.