Obviously just a guess, but if Roman has a guy that he trusts that much, he will likely make him his OC when he moves on. So at best the Bills are getting the 3rd best guy. Anyway, we've seen that we can move on from great coordinators with replacements (Pettine/Schwartz), but this is the first really good offense we've had in a really long time...it's going to be tense losing Roman.
Rex said this is his last HC job, but he didn't say he wants to retire soon. I assume as long as he's doing well and not getting fired, he'll be around for a long time.
Money? It's great having this much talent on the team, but we just simply can't afford to pay everyone. Glenn, Incognito, Harvin, Gilmore, and Tyrod Taylor are all going to want bigger extensions. If Taylor keeps playing like he is, he'll max out anything the Bills have all by himself (and that's already with the assumption that Mario is a goner).
Even though it will suck to lose some of these guys, these are good problems to have. We're just not used to having them.