I just thought of a big reason why it wasn’t a demotion:
It would absolutely SUCK to be one of the Q12. You’ve got the weight of the world, everyone is always watching and scrutinizing everything you say or do… and being in the first presidency makes it even worse. Adding to it, I suspect Uchtdorf might disagree in a few key areas with RMN on how to lead the Church, which would make it even more difficult if he remained in the FP. I like Uchtford enough, that I almost don’t want him to have to deal with that added struggle.