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?
It would be nice if they could figure out how to get a Quad Core processor in the Surface Book somehow. Not sure how they would pull that off given the thermal constraints of the QC Core CPUs, but that would be a nice differentiator between Surface Laptop and Surface Book. Microsoft doesn't have anything that quite matches the MacBook Pros (15").
That's why I have a ThinkPad P50 in addition to my Surface Pro (2017).