Is there a scenario in which an area will be completely clear of Captains? Will regular grunts keep replacing slain Captains?
Using experience from Shadow of Mordor, grunts would automatically fill vacant spots without notification and time advancing methods. I trialled this quite extensively to my dismay.
I've only briefly played Shadow of War (restricted to Minas Ithril) but I did notice a new captain replace a fallen captain without entering any cutscenes via missions or dying.