I kinda hope they don't do that. There are some benefits to being one piece, but it better be an amazing design. If not, the lack of another wow factor would be unfortunate
I say go for it as long as there is also a version of it that DOES have a detachable screen, or maybe just make a dock for the new Surface Pro 5 that turns it into a Surface Book... Actually, yes. Do that.
I highly doubt that Microsoft would release a 'me-too' product like a regular laptop. BUT I wouldn't be so sure that Microsoft wouldn't merge the two families (Book and Pro) into a single line of devices where you can pick and choose the power and mobility of your Surface. Heck, it would be an easy way to streamline a Surface Phone into the mix if that were the case (obviously not in the near future, but one can hope)