Whether or not the cast sheet is full, if it was written from day one or if it "just happened" the simple fact of the matter is... It's a Zombie Apocalypse... If circumstances pop up, Michael could die. A main character can be a victim anytime, especially due to growth. You have a solid ten people constantly making decisions and working together each of those ten have the possibility of stepping up anytime.
The RILEY character IS a Leader and could run it all. CJ is definitely someone Michael would have stepped down for. The Angel character "could have" taken over.
The fact of the matter is ALL BETS ARE OFF when survival is a factor.