You could, but not without sacrificing many of the features that make the Surface Book unique. The pen support in Windows 7 is significantly worse, and face authentication wouldn't work. Windows 7 isn't very usable on a tablet. To use the GPU when you reattached the tablet you'd likely have to reboot the machine, also a much slower operation on Windows 7.
If you don't want the above features, you could get a machine that runs Windows 7 well with good hardware specs for less money.
If you don't want the above features, you could get a machine that runs Windows 7 well with good hardware specs for less money.