I think there's a huge difference between "maybe there is a backdoor" versus "literally all of microsoft, across all orgs, is owned and they have to shut it all down and start from scratch", call me crazy.
(… especially when you're not even bringing up the fact that the compromised key was mainly usable to access e-mail)
I don't know what your point is.
If an attacker had full root across the org for an undetermined (but not short) period, I'm unsure what other approach you think you could take? You can't just run MalwareBytes and call it a day.
This comes down to a risk assessment. No company has a breach and just shuts everything down, that is insane. When we perform IR we build a detailed timeline, we collect the scope of potential access, and we form a remediation plan. We don't just go "well hey, anything can happen right? shut it all down".
So the question is how you handle such a situation.