Actually Rollins wouldn't.
I liken this to the Batista/Orton situation from 2004 (the difference being the heel/face dynamic: the heel dynamic is far easier to play off).
Rollins is chosen as the heel incumbent, but it would be Reigns stealing Rollins, fire so to speak.
It would be a happy accident that WWE would be forced to milk.
Somewhere toward June's expiration date, Rollins would attempt to cash and fail. By then, he'd have drawn enough sympathy from the crowd to begin a face turn.
My argument states that if Reigns did turn heel, he'd be given Heyman and a fairly long reign. Whomever is most over well into Reigns' reign would take hs a** down.
Well, if/when Reigns' heel turn comes to play and Heyman (possibly) joining him, they'd better make him look credible and badass, although it's gonna be hard, knowing that he's got a limited personality and not so great in-ring ability. Not shitting on Reigns, but he does need some versatility in his personality and in-ring work. If he's indeed their 'golden boy', though nothing's been set in stone yet, then they should invest in him with everything they've got.
I would absolutely hate it if Rollins' MITB cash in wasn't successful. Doesn't matter who the champ would be during the cash in, but it needs to happen.
I've said this before and I'll say it again, it'd be so awesome if they let Seth cash in on the future champ at WM31 (Reigns, quite possibly). But I doubt that's gonna happen.
Best case scenario, WWE somehow manages to pull their heads outta their asses and decides to push the much more talented Ambrose (Rollins is outta that picture, since Rollins will be remaining heel for the next 8-10 months, give or take.) We'll see what happens in the end...