What I see here with the X1D is the second enterprise (after Leica) to create a fully integrated MFD system (one OVF, one EVF), good-bye to good old modular concept which traces its origins back to film era, where interchangeable backs made a lot of sense. P1 (with Mamiya and Leaf), Hasselblad and Pentax share this slow and evolutionary transition process which worked as disincentive to disruptive thinking.
This is kind of sad for tech cam guys but I think the days of separate digital backs are counted. Same holds for modular viewfinders (the new waist-level finder is called articulated LCD with touch functionality). And if you ask yourself nowadays why the H/XF/Z have to be that big and heavy as they are, you should be looking forward to theses changes.
As with Leica's S system you mention I think a big issue isn't just that the system is integrated rather than modular but that its designed from the ground up for a smaller medium format sensor size. The modular systems and Pentax's digital 645 all use flange distances designed for full 645 film which makes them a lot deeper and with the modular ones also with much larger prisms.
My guess is Hassleblad probably see the larger 54x40mm sensor size as the future for the H-system and cameras like the X1D as the future for the 44x33mm sensor size that can lead to a smaller system if designed from the ground up.
With the smaller sensor size as well I'm guessing theres potential for further improvements in resolution too with the lens performance to exploit it, if 35mm digital pushes up to 60-70 MP I'm guessing lenses will start to become a big limiting factor.