Same applies to DSLR and P&S, 4/3 etc - the bottom end ie 600D is so much better than the 400D for example. And so the MF will have to continue with improving their bottom end. The Pentax 645 must be due for a major revision soon - perhaps 60mps and 2fps?
*nods* that is where I was going with the idea of more choices causing issues for some companies. When there are a few choices, most fit poorly, and many people go with something that is less then ideal but is as close as they can get. MF manufacturers currently use this to get customers who are not really impacted by the best features of MF but need something that DSLRs do not have.
The same thing is eating into DSLRs from the other end. Mirrorless cameras are not 'better', but as they start to overlap DSLR capabilities, people who were only using DSLRs because they lacked a better suited option will migrate to them.
Now, the real risk to some MF manufactures is their lack of diversity. If you look at Canon/Nikon/Sigma/etc, they produce a range of cameras so if markets shift they can shuffle production... in other words they can cannibalize their own sales rather then have others eat into them.
Some MF manufactures can do this.. Sigma has a nice broad range of photographic cameras, Megavision has its scientific imaging and other specialty offerings.. but PhaseOne or Hasselblad? If the MF market shrinks then they shrink (though I could be wrong there in that I do not know if either of those two examples actually do have other markets)