The most important part of that being that Jimmy actually had to deal with salary cap, and there for roster restrictions, that he never had to deal with in Dallas. Same for Bill Walsh, I agree. Had he been around during salary cap times you have to wonder how well he'd have done since he built the Niners with teams that were loaded with talent cause they didn't have salary cap restrictions.
I'm not saying, and never would, that Jimmy wasn't a great coach, and that he doesn't deserve a lot of credit, but the Miami situation proves that it wasn't all Jimmy and that dealing with restrictions that the salary cap brings was not his strong suit and hurt him a lot. I'd be willing to bet that is a huge part of the reason he's not ever got back into coaching since he left the Miami gig.
Now on the other hand you have guys like Bill Cowher, who dealt with salary cap through the majority of his head coaching in Pittsburgh, and while they weren't always knocking on the door of the Superbowl his Steelers teams always seemed to be competitive and he always seemed to do a very nice job of getting the most out of what was given to him by his owner(s) and GM.
I'm not saying that to say that he's a better coach, by any means, just that I do believe some people are suited for the game the way it is now and some that simply are not. As much as I like Jimmy, and loved having him here, I simply don't think that the league, with a cap and those type of restrictions that really puts on the amount of talent you're able to get on a roster, is one that Jimmy could be successful in.