Canon and Nikon have only one -but invisible- asset - user inertia motivated by user's belief that it is cheaper to keep using their old lenses than to buy into a new system. Bringing in mirroless quickly would be like puncturing a cellophane wrapper - once punctured it desintegrates.
Well, there is inertia, but there is also...
- the fact that today high end DSLRs, such as the D810, 1DX,... remain the best image making machines available in terms of image quality and shooting enveloppe,
- the best lenses are only available in EOS/F mount,
- Profoto and many other key "accessories" manufacturers only ship their most appealing hardware for Canon and Nikon (think B1,...),
-...
Today, the only tangible advantage of an a7r I see is compactness.
If I were to start from scratch today I would buy exactly the equipment I currently own.
Cheers,
Bernard