If DSLR's are to be replaced will cell phones, then why is there so much interest in FF and medium format cameras?
The interest is largely in the echo chamber of online discussions. The market for DSLRs as it stands is far larger than the enthusiast community online and chatting about the various merits and demerits of cameras.
A huge amount of this market is soccer moms chasing their kids around the house. If a simpler, easier and more convenient alternative presents itself she will use it.
The sales numbers speak for themselves. DSLR sales are in decline albeit slower than P&S but they are not increasing. The costs will have to be recovered at some point and in some way. Layoffs, plant closure, R&D spending cutbacks seem to be the usual recipe for this sort of problem.
Making something risky and disruptive is not usually part of the business plan of most companies however, to their credit, Sony is a lot more adventurous in this regard.
But I would also note that the biggest change to the camera industry was the phone and not a FF or MF camera.