Great question, Douglas. The guys over at LuLa are wondering about that too. Especially why they (Sony, Phase) didn't go straight for a stitched D800E/A7R sensor. Perhaps fabrication difficulties, but then again maybe it's just marketing. Make 'm buy the 33 x 44 mm 50 MP now, have 'm upgrade to a 'FF' 75 MP next year.
A super sized A7 that would, with adapters, take Contax, Leica, Phamiya, Pentax, Hasselblad and all forms of medium format lenses would be nice, preferably with AF and fully functional aperture, etc. If Leica can reverse engineer the protocols, surely Sony can too.