I agree and thought of that a couple years ago. The issue is that it takes a loong time to converge APIs of desktop and mobile. Plus, there are power and cooling issues. Next, stuffing in a CPU and GPU that can satisfy both requirements is basically asking for completely new cores that can do high power and low power. Cooling for 3D desktop games would require tradeoffs.

The issue is whether it's worth having a bicycle that can turn into a car and a car that can turn into a bicycle. Most of the time, the execution on these is terrible at both.