Well don't forget Surface Book is getting quite old now (yes, there's the performance base, but the whole idea of it is the same as before) and it might be a sign of what's coming in the future. Probably they're going to change the whole marketing behind the SB in the next version, something like the ultimate laptop because of dGPU? Or Book might be discontinued as we now have "the perfect Pro" and Laptop?
We have to wait and see!