For a number of years now people have been predicting that mirrorless will take over. I think this year of 2018 will be the big test.
Digital SLR sales continue to drop. Mirrorless continues to grow. With the growing restrictions on carry on luggage on airplanes a smaller lighter camera makes a lot of sense.
A little Canon EOS M5 now makes a better camera in all regards to the most basic model DSLR, and is smaller and lighter. Olympus have proven with their OM-D E-M1 that mirrorless can be professional, and if you want to have the full frame verses small frame argument, then try a Leica SL mirrorless full frame camera with a viewfinder thats the best in the world for EVF. If your wanting big you can now even go medium format with a small light mirrorless body.
If Nikon and Canon do release real mirrorless contenders this year then within two years its game over for the DSLR, and it will be come a niche player like the rangefinder and TLR (Twin Lens Reflex) before it.