I was a bit surprised to actually see Ambrose retain. I'd have expected interference on Reigns' behalf (perhaps Triple H screwing Rollins, turning him face and Reigns heel) to put the title back on a now-heel "wellness policy violator" Reigns under protection of The Authority (or what's left of it)... but apparently, WWE didn't want that kind of bad press either.
So seeing that, keeping the belt on Ambrose was a smart move. Rollins is over as it is, and doesn't need the belt right now. Reigns is WWE's pet project and will be kept in the main event no matter what. So Ambrose was the only one not considered quite on top tier level, especially after some more mediocre performances on the most recent PPVs. Had he lost the belt now to either Rollins or Reigns, it would have served only to further establish that he's "not on their level" and, in a fair fight sans MITB briefcase, would come up short against either man anyway, thus effectively relegating him to permanent midcard status.
With him now winning clean, however, he's kept in the main event picture... and while he might not be as over as Rollins, it's certainly a step into the right direction. If WWE wants to invest into Ambrose, this is what they have to do. I'm pretty curious about Summerslam now, particularly in who Ambrose will face for the title. I don't assume they will go for another triple threat situation with Reigns and Rollins, so I could imagine a Reigns/Rollins feud for "undisputed" #1 contendership, culminating in Summerslam, while Ambrose gets another opponent. Traditionally, the World title match at Summerslam is a pretty big one though, so it should be a top tier level character.
Question remains: Who could it be? John Cena is always a choice, but it would face vs face. Also there's still the AJ Styles feud going for him. Randy Orton and Brock Lesnar are both occupied with one another, Reigns and Rollins too (if they have that contendership feud, obviously). Bray Wyatt might be a choice, but he doesn't have quite that same star power... So maybe it will be Reigns or Rollins after all, though I somehow don't see that. Outside chances are for a Triple H appearance or Undertaker appearance, though the latter is highly unlikely, and Taker wouldn't return at Summerslam just to lose to Ambrose, I'd say.
So unless WWE pulls another (part timer?) rabbit out of their hats, I'm currently a bit at a loss at the question for a possible decent WHC title match opponent for Ambrose, outside Rollins/Reigns.