Assuming you were planning on getting a Surface Pro to do real work on (typically requiring a desk, keyboard, etc.) can't you connect an external hard drive to it?  External storage space is extremely cheap.  If you're making the argument that it should come with everything it needs to "just work" out of the box and be ready to go, I can respect that, but I don't see how you could call that a laptop/desktop replacement.  When I personally think of laptop/desktop replacement I think of at least something resembling a docking station: external monitor, keyboard, mouse, et al.  Part of that would have to include an external hard drive for me.