If Rogue represents that big of a cost savings. You could be right. If the performance is in the ballpark, and it's that much cheaper. Although I've read high end GPUs have high profit margins, so I think if any GPU company wants a console contract, they can bid and still earn a profit. I'd think Nvidia would take the work to stay relevant. But maybe Sony had enough. We'll know soon enough.
I haven't been following the economies of fabricating the latest GPU silicon. What you say makes sense. And Sony could use as many cores as needed to equal or exceed whatever will be in the next Xbox system, as needed, depending on their strategy. If these Series 6 chips are that efficient per mm2 and watt, it could prove to be a prescient move. Due to how costly console launches have become, I think the next gen will likely be long like this one, I hope neither company cuts too many corners. If a 16 core Power VR6 equals a 1080p optimized HD7850 (hypothetical next gen AMD console GPU), maybe they could throw in 20 cores for the same price of AMD's chip due to the cost savings.