Rollins is basically out of options by this point, and Cena could do an awesome job with the young heel talent the WWE has while being a dependable lynchpin champion while people continue to progress and develop.
Rollins is only out of options, because the WWE doesn't know what the hell they're doing with Owens or Cesaro. A triple threat could certainly work. Obviously neither guy would get the belt, but it would be a solid match.
It also makes it easier to vacate the US title in a "place of prestige" and hold a tournament to see a new champion. OR perhaps they simply absorb it into the WHC and go with just two singles belts.
If he vacates it and they have an extremely competitive tournament for it, it doesn't lose much steam at all.
I honestly hadn't thought of them just vacating the title. I don't see why Cena would spend all that time with the open challenges, and then "Eh. I don't need it. Got this fancy new belt." Even with a good tournament/match to crown a new champ, there will be that age old argument of "Well, they never pinned the champ."