Yes, they do. So far highest ratio of compute-graphics on DX12 games is 20:80. And There will be titles where everything might be procedurally generated(including textures, objects, etc), so it will have 80:20. What will happen then, when Nvidia GPUs are not able to switch context between graphics and Compute?
So far none of games pushed DX12 to the absolute limit.
Second thing: Do you think it is logical to buy more expensive, and worse performing GPU? Which is the case of Nvidia? When you can get the same type TDP GPU with similar type of performance for less? Remember, R9 380 is almost on par with GTX 970, uses similar amount of power but cost quite a lot less.
GTX 970 is 340$. R9 380 is 190$. See the point?