True. But spear phishing is also harder with this standard, since you can't just phish credentials; you need to actually compromise the user's authentication device. (Get root on the user's phone, physically steal their authentication token, etc.)

In this thread, you keep moving the problem statement to cover a smaller and smaller subset of the general problem of account compromise. This is exactly what the attackers you're describing will do.

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.