If there's a CVE vulnerability that is being actively exploited on your network, you should preempt running processes to deal with it, and absolutely must take the boot+nuke approach, because it already could be affecting any host that has not already been boot+nuked?

If there's not a CVE, AWS can significantly manage the lifecycle of their machines, and have ~5% of all of their machines "unschedulable" at any one time, waiting for existing processes to complete so that they may use an orderly restart before doing a boot+nuke. A SLA of "Tasks may never run longer than X days"(x=10-30) allows them to perform orderly restarts.