From what I've read in this forum and elsewhere online, and heard from camera-club friends, the major drawback to mirrorless at this point is the electronic viewfinder. Either it lags a bit or it just doesn't feel right compared to an optical viewfinder.
Another point against has been ergonomics, the comfort of holding and using the body when various lenses are attached. Some feel Sony's FF mirrorless bodies are too small. (As to this, there was a time when smaller smartphones were being marketed, but as more functions were crammed in, overall size, driven by screens and user demand, began to grow again.)
With consumer feedback, marketing savvy, and clever engineering, the negative aspects of mirrorless will be greatly reduced. Technically, perhaps, an EVF that truly satisfies and wins over longtime dSLR users will be the toughest challenge, but it will be overcome.
Which leads to the question: Do Nikon, Canon, and Sony plan to phase out dSLR's, or are the big three approaching mirrorless with uncertainty? If Canon and Nikon create FF mirrorless that win over the vast majority of professionals and dedicated enthusiasts, what incentive do they have to keep producing dSLRs?
With one or two generations of FF mirrorless winners, dSLR's could be entering the sunset phase of production. This could be as soon as seven years.
Personally, I predict dSLR's might then be going the way of vinyl turntables. Available, but only as a tiny niche product that can't be supported by the big three. (I foresee a time when Canon licenses some dSLR lines to Chinese and Indian manufacturers who can make a small profit off a dwindling volume of sales, dependent in part on used and cheap third-party lenses.)
Perhaps a small quantity of professional bodies, priced very high to be worth producing, would still be sold to a handful of sports photographers, but I think the EVF will soon reach the point where it is superior at tracking action. The market for rugged dSLRs in the hands of journalists is evaporating, as print publications are disappearing, and many news organizations have already dropped staff photographers altogether.
The SLR has been one of the most successful masterpieces of design in any industry, with the first 35mm version appearing back in 1936 in Germany. It has made the amazing leap, with few mechanical changes, from analog to digital. The age of a technology doesn't predict its end, as can be seen with the internal combustion engine, but it does allow time for competing technologies to develop, outperform, and out-sell.
Once the EVF and ergonomics of FF mirrorless are as good or better than what a dSLR offers, why would the big companies keep producing dSLR's? Beyond a tiny subset of photographers who cling emotionally and/or habitually to the familiar, older tech, what would be the market?
