If an ad can use DoH to sidestep a firewall, so can an employee. If Google and Facebook were cunning (and nefarious, but that much is presumed), they would be aggressively developing a product that solves this problem for corporate networks, but at an enormous cost. Otherwise, when corporate networks solve this (and they will), home users who hate ads will just follow whatever pattern they settle on.

Encrypted DNS is going to upset a lot of corporate network management/monitoring... But the switch to HTTPS for everything caused similar frustration for IT admins awhile ago.

In the corporate world, I think the future is managing your network by managing every single device on your network. Only let authorized/corp devices in and all those devices must be enrolled in an MDM solution that enforces all sorts of policy and includes monitoring traffic/DNS queries. Of course that's a lot more work than just monitoring things at the network level.