I think the bottom line is that MF isn't going any away. Pentax, Leica, Blad and Phase/Leaf may ultimately lose the pro market to the D800 and its successors, but there are plenty of people who want the ultimate in anything, be it a scarf, jacket, lens, car, or imaging device. Hi end audio or the stupid expensive car market isn't going to disapear just because Audi/BMW/Porsche/M.Benz make cars that are in the same league as a Maybach/Buggtti/McLaren, or that NAD amps are good enough so McIntosh will be gone. There is a reason that VW/Audi makes the Bugatti and Benz makes the Maybach (which is possibly the ugliest car I've ever seen, like a Ford Topaz crossed with gigantism), and that MacIntosh tube amps are still being made. There is always some one who will pay for the "Ultimate [Image] Quality".
In my personal opinion the differences in IQ between the D800e and the older generation of backs is so slight that it doesn't make sense to use the older backs, when the D800 not only offers the similar IQ, but also offers flexibility to get a shot that you couldn't get with a back. Again, I'm talking about the older generation of backs, like the Phase P25/30/45 plus series and the Leaf 54s/75s. Not that those backs are bad, just that the Nikon is that good. Before I get flamed I'm not bashing the backs at all, especially the new Dalsa based backs of which I have no experience.