K, you're right, replacing both is a bigger challenge. However, just because a younger guy played less than the senior, that doesn't mean they aren't as good or are equally "adequate".
Beanie Wells was limited in carries because Pittman was the older guy. Moreno redshirted at Georgia simply becausw they had older guys they thought could get the job done. Terrell Pryor started the year playing a few plays at a time while the senior Beockman started. Since they weren't redshirting Pryor, they eventually switched to him. Even Bo waited his turn to some extent.
I'm on my phone, so I'm done with the examples, but you get what I mean. Just because a younger guy doesn't play much, it doesn't mean they aren't equally capable...sometimes it just means it's not their time yet.
Adrian Peterson and Herschel are pretty rare. Most guys, even the Bradfords, Vince Youngs, and Leinarts of the world waited behind guys whom they were better than.