Nothing lasts forever, but they can sure stick around a long time. Look at Leica... they carved out a niche and are doing ok out of it. Even *if* mirrorless takes over the market served by lower level DSLRs, I think there's enough niche for higher end DSLRs to survive for the foreseeable future. If you need digital full frame, there's only 3 real choices at the moment, Canon, Nikon and Leica. I'm leaving out Sony for now as I believe their full frame models are discontinued pending replacement. Canon and Nikon are both rumoured to be bringing out cheaper full frame bodies so that will help too. Maybe, some day in the very distant future, there'll be more mirrorless full frame systems too. But it might be too little too late as "good enough" of smaller sensors will do for the masses, leaving full frame as another Leica-like niche.
Then there's also the chance that a future technology renders what we know as photography right now obsolete.
My guess is that if a mirrorless FF system does appear it will be more in the image of Leica(mostly primes and maybe a few smaller slow zooms with limated range), theres no way high quality zoom lenses would ever balance well on it.
Also as far as sensor design goes my guess is that optics are going to increasingly become a barrier, the most recent 24MP ASPC sensors can't produce maximum boarder to boarder sharpness on anything but a handful of lenses. Optics arent advacing at anything close to the same rate that sensors are those advances come at the cost of ever higher prices.
If you want boarder to boarder sharp 40 MP images my guess is that your going to need to go FF for the foreseeble future. I think theres a significant market for that kind of resolution aswell as the D800 has showed when the price is at a reasonable level.
Canon seem to be looking more at the reduced pixel count for bigger photosites.
My guess is that this is partly linked to optics, if you go much beyond 18 MP I think you'll see flaws in alot of there current lens lineup start to stand out much more clearly.