Why would they market the new system to their own fanbase? They know the vast majority of people that own a cube are going to buy the REV. Putting backwards compatibility may help get the rest of the stragglers, but in terms of sales that doesn't change things much. Backwards compatibilty's main purpose is to continue the sale of GC software throughout the REV's life (see PSone). This is only possible if you don't need the previous generations controller to play. It is the only way to successfully stretch out the Cube's life. Also, if Nintendo decided, they could make a portable GC which would make the REV ultra appealing. Why? becuase they can just take the disk out the GCP, plot it into the REV, and continue playing.
It seems very likely to me, but the first step must be allowing the REV controllers to play GC games.