There is a big difference between being the number 1 back and the backup though.
They have the same yards per carry as you mentioned but how many more carries for McCoy vs the rest?
And do you think those guys can handle 15 to 20 touches a game each? I don’t.
Also, there is a big difference when teams game plan when you’re the starter. When shady is in, the the defence will game plan for him, not the backups of course .
The best example I can think of was Karlos Williams. Great backup RB for that year and a half. But when shady missed time and williams was the “bell cow”, he was very ineffective.