Yes, I know that Reagan was a "bigger figure" than Romney and Huckabee are now. The point is that every one of these cases is unique. In 1996, you could have argued that Dole would break the pattern of "runner up from last time around gets the nod" because he was so old. In 2008, you could have argued that McCain would break the pattern both because of his age and because of the party base being unsatisfied with him. But both times, the pattern held.
I don't see why being out of office for six years is a good enough reason to discount the "runner up from last time around gets the nod" this time around. Why would that be a big factor? It seems to me that, nowadays, the #1 way to get onto the fast track to top tier status in the primaries is being famous. Huckabee and Romney are now more famous than Sanford or Crist or Pawlenty or Thune. Unless one of those folks is tapped by McCain as VP, Huckabee and Romney will still be more famous than them a year from now and two years from now and three years from now, and will have an easier path to top tier status in the 2012 sweepstakes.
You crafted my response in your own... when in doubt, Republicans run to experience. If Rudy had been a two governor and a Senator, they would have run to him, probably.
The problem that Huckabee and Romney have is that only the modern climate could have seriously justified the candidacy of either of them. If there had been an obvious, experienced conservative choice in this race, they wouldn't even be footnotes.