This already happens - it's called spear phishing. Instead of wasting time with multiple users, the attacker picks a high-value target. For example, if you can get one admin, it's game over.
That's the point of this solution. If we can remove the vast majority of account-compromise tactics and force attackers to achieve success only if they "get one admin," then that's an incredible victory -- especially for the large number of ordinary people who are not admins.