If a single click on a phishing email can ruin the entire company, the blame doesn’t lie with that individual.
Yes but not every user needs access to every system all the time and there should be alerts set up for logins outside of working hours, expected devices and IPs. There should be behavior based alerts, for example, why is the HR lady opening PS?
There are many things that can be done to secure the systems post-compromise.
Oh, of course. But that’s for compromises utilizing tool chains and exploits you’re aware of. Zero day exploits are commonplace nowadays and often utilize complex tool chains to avoid detection or circumvent security posture. It’s all a matter of how sophisticated the attack is and it all becomes a lot easier to do if you’ve got user level run permissions due to some user clicking a phishing email and tossing their creds in it or launching a random pdf with an embedded payload.