The email servers are supposed to be archived along with an audit trail that shows what changes were made, when, and by whom. That's pretty much standard practice across the board. The claim that it would take years to revoker the emails is laughable - it's not an instant process, but if took more than 2 weeks, they're intentionally dragging their feet.
Actually, with some internal search features, it is almost instantaneous.
Some of the government e-mail solutions are custom written...the Clinton White House email system was notoriously so. As such, they have archiving functions that don’t allow unarchiving in a competent manner. I suspect that it is at least partially on purpose...or at least opportunisticly unfixed.