Yeah, this is largely my thinking as well. But then again, until last year the Undertaker losing at WM was something that simply didn't happen either.
That said, since Reigns is the E's chosen one, I expect they'll be desperate to find a way of getting the strap on him. I'd more confident of Reigns winning than I would be of Bryan winning had he won the RR, for instance.
Having thought about it, I can kind of see where the E were coming from with their Rumble booking - they just horribly miscalculated how much Bryan was over, and how Reigns comparably is not (which is pretty ridiculous, all things considered, but that's the only way it can be rationally explained imo - other than it being a case of Vince basically just saying "**** you I'll do whatever the hell I want).
They obvs anticipated the huge support Bryan would get for instance, and the decision to have him eliminated early was probably a calculated move to get the fans riled up early, and get the booing out of their system. They were obviously banking on the elimination of Bryan being compensated for by Reigns emerging and taking centre stage. They probably thought Reigns was over enough to sate the fans' anger, which was obviously a terrible prediction. The decision to leave him with Kane and Big Show at the end was also obviously done to eliminate the possibility of the fans rejecting the overall winner, which I suspect they were worried would happen if it came down to Reigns and Ziggler or someone - this also obviously totally backfired.
Rock was then obviously used as the trump card, who would really get the fans on the side of Reigns, but even that was not enough. The whole thing makes for very bad reading, and was obviously based on horribly flawed logic and assumptions, but I at least can see where they were coming from (if this is indeed the way they went about things), even if that way was utter trash.
Will be interesting to see if Rock is on RAW.