Even if Reigns loses to Bryan, there's been examples of Rumble winners losing their spots before and earning them back before 'Mania (Rock in 2000 and HHH in 2002, Mysterio (2006), not so much because Teddy handed it back to him)
This is for the best. This will allow Reigns to have a quality match with a reliable pair of hands and Bryan will do his best to elevate the lummox. It also allows for a passing of the torch from one 'Mania headliner to another, and the fans will be split and invested in their chosen winner. It makes for a strong and believable story that blurs the lines between fan's grasping of kayfabe and reality.
Think about it, the question here is are you, the consumer, on the side of what the machine wants, or the side of who the people want?