I dont think "prime" factors in, as a running back. I mean, he could be Priest Holmes, come September 2007, hurt himself, and all of a sudden, he is no longer the Tiki, we know.
31 is up there, and he isn't a running back, that has little problems, taking a big beating (some can deal with it better then others)
The one flaw, was announcing it during the season. He gains very little from doing that, simply because he knows he will be hanging them up, but to bring all the spotlight, was kinda pointless. But thats just me.