I was surprised to read this assessment from Mike Johnstone, Online Photographer,
The Online Photographer Quote: Many have said that full frame (24x36mm) is taking over (2012 was the Year of FF, no question), but I think the opposite: Micro 4/3 is almost too big for most people and most uses. FF will die out, I think, except as the "medium format" for professionals, and the public will end up with smaller-than-APS-C sensors in the cameras it uses almost exclusively for video and stills in electronic display formats. I just really hope that the eventual standard will be something as large as 1-inch.
I don't believe anyone out there knows how the future will work itself out. Mike is not only saying that there will be a lot fewer FF cameras, but fewer APS as well "and just hopes that the eventual standard is as large as 1 inch".
Overhead costs of production go down as more customers buy into a product. I don't think personally that FF cameras will take over the majority of DSLRS - they are about 9% of DSLRs now, i think. But who really needs DSLRs with FF or APS, there's only a small percentage of professional photographers out there. For the rest of us, its art or a social activity that drives us, not an activity that is central to our survival.
So are there enough professionals out there to sustain FF by themselves - i doubt it. But there probably is enough enthusiast interest to sustain FF where it is today - about 10% of the more professional cameras - so called DSLRs. IMO. Can't see APS going away anytime soon.