Erasure can be done much more efficiently with crypto erasure, where you encrypt the user data and just delete the key when users request deletion. This way you don't need to spend expensive tape vault / library time rewriting a tape.
how does that work 'down the road' in 10 or 20 years?
By that time you've lost the key so the data is already deleted.
Do you mean the encryption might be broken in 20 years, or how do you keep the key for 20 years?
AFAIK, symmetric encryption (which is what you'd use for crypto erasure) is not thought to be vulnerable to quantum computers, and it would be a huge mathematical breakthrough if it turned out that currently-known symmetric crypto algorithms were actually weak.