I have that idea before. But, it is a confusing product creates in-fighting with its existing product offerings.
SP already has kickstand and a sturdy keyboard, if not completely solid, but, the thiness, the weight, the texture, the snap are vastly more appealing. Making a solid base sends mixed messages to consumers.
SB like other said, you don't need the kickstand because it can just turn around and dock the keyboard. And SB tablet mode isn't meant for long use compare to SP. The screen is too big, creating too much torque on the hands. So, SB is not meant to be a real tablet. There is SP for that.