D800 -- end of medium-format?

You might wish to see this article:
http://www.lensrentals.com/blog/2012/03/d-resolution-tests

Notice the verdict of the comparison of the 800 to Hasselblad:

"For the couple of people, though, that seem to think the D800 is a medium format camera in 35mm clothing, I would point out that a Hasselblad HD50 with kit lens tests out at about 1,600 lp/ih, so no, we’re not quite there yet."

I think it's difficult to make the argument that MF is obsolete unless one is a professional. As a professional violinist, it's hard to convince the layperson that a $500,000 violin is really 10 times better than one for $50,000, or that a $20,000 bow is worth 10x more than one for $2000. But for many, they are, if even for what most would consider marginal differences: slightly better projection, slightly better response, a little better palate of colors. For certain professional photographers, I think the same applies.

I'd also add that for most of the MF systems (except Pentax), digital backs can be swapped out as better ones appear--that's not true of Nikons or Canon. And as technology improves, there's no reason that the larger sensors will fall behind.