Ah ok, I see.
2 problems with that then. Firstly, I thought it was universally agreed that F1 being aero dominated was a bad thing? And secondly, at least there are multiple teams running Ferrari and Mercedes engines, whereas there will only ever be 1 manufacturer running its aero, so dominance will be at an individual team level.
With identical chassis and custom engines, you could expect all the Mercedes powered cars to be fighting for wins with Ferrari powered one nipping at their heels.
With identical engines and individually designed chassis, you may as well give all the trophies to Red Bull and not bother with the hassle of running a Championship.
Also, its worth pointing out that Mercedes and Ferrari don't have a strangle hold on the rules through being engine manufacturers. The engine manufactures are actually totally absent from the F1 Strategy group. Mercedes and Ferrari are there as teams, along with RBR, Williams, McLaren and Force India.