I believe it has to do with cubic capacity. A single C-5 can transport 2 Abrams tanks plus a bunch of other stuff, a C-17 can only carry one and not so much extra. Remember, the C-17 is between the C-130 and the C-5 in size.
Yes, I’ve been on all of the above, I understand. Heck, I remember playing with load floor planning paper dolls when my dad was at staff school, many, many moons ago. How many AH-1s can you cram into a C-141? Heh.
Now, those numbers you quote argue for much less than a 10:1 replacement ratio. 10:1 sounds like something a Boeing military sales rep would come up with, is all I’m saying.