It is not difficult to see how continued intransigence on the part of the White House (and if you think the White House isn't pulling the strings, well...I've got a bridge to sell ya) could produce impeachment hearings. Also, remember that electronic information is NOT stored merely in email files and on hard drives. Hard copies are made, but also transaction logs are stored and backed up on servers. With enough technical resources, these logs can be read and their contents discerned.
I would not want to be one of the lackeys destroying hard drives and shredding docs at the boss's behest knowing that I might be putting my own butt in a sling concealing evidence that is readily discoverable by other means.