Note that I don't know the details and am making assumptions that may be wrong about the case in question, but in general, if you can't deny access quickly to any given account, you really want to fix that. Not just because of rogue ex-employees - what happens when $important_person's account is compromised?
No. It's easy to revoke access to a user. An admin is different - an admin can install whatever he wants to give him backdoor access. Or a timebomb.
Detecting unauthorized software from a rogue privileged user is a different problem with very different mitigations. It is a great topic that I'm personally interested in, given that I'm implementing controls for that, but I wasn't discussing that.
If I leave $20 on the sidewalk the thief is wrong for stealing it, but it is partially my fault for being stupid enough to leave $20 on the ground.
This can easily happen to anyone -
5PM Friday - $admin and $ceo have a fight
6PM Friday - $admin decides he's had enough with $company and $ceo, and wipes everything
7PM Friday - $admin is fired
Where are your exit procedures now? Are the current admins stupid for not having the foresight that $admin and $ceo's fight would have resulted in the worst?