This is the Emmitt - Barry debate sorta.
Zeke is a power back with good straight line speed. He’s an elite back. Whether he’s the best is debatable. You put a Barkley or even a McCaffrey behind our line, you don’t get the same power, but you get more wiggle, a WR type RB, and more HRs. But you may lose some 3rd down conversions. You may lose the concept of wearing a defense down and DBs who don’t want to stick their heads in there in the 4th quarter.
The point is that any RB can be replaced if your OL is good. Different RBs give you different things. There are probably at least 10 young RBs in the league that would start for us and produce big time, and we would still be winning. But they aren’t all “as good” nor do they all have the same traits. Kamara is definitely NOT a Zeke, but he would be fantastic in this offense.
The fact is that unless you bring in a RB in the 3rd to 5th round every couple of years, you won’t have that great backup that shows you you’ll be okay to let Zeke walk. You need to have that guy on your roster ahead of time.