The problem Nikon and Canon will have deals with the importance of the sensor in mirrorless. Canon is way behind Sony's sensors and Nikon relies on Sony for it's sensors. Unless that changes dramatically Sony will have the advantage. Frankly, I think that is one reason they are clinging onto DSLRs so much and hoping mirrorless is a fad.
Canon's sensors are underrated by DXO scores... They really aren't thaaat far behind. And Nikon actually uses sensors from several makers. Canon's sensors are indeed "good enough" that it really wouldn't be a limiting factor. Plus, they can always buy Sony sensors too, which they have started to do with the G7x.
I don't think they believe that mirrorless is a fad... More likely, they don't want to disrupt their client base. Sony had so little dSLR market share, they could afford to take a big risk, and launch a whole new mount system.
Nikon has not launched a competitive APS-C/Full frame mount system -- instead going with a 1" sensor mount for their mirrorless. Plus, the earlier models especially, were totally anti-enthusiast. No manual control dials, etc.
Canon has launched a APS-C mirrorless mount -- but the newest model hasn't even been released in the USA. It lacks a viewfinder, it only have 3 or 4 native lenses.
Their fear.... If they aggressively launched a new mount system... It could interrupt their current traditional lens sales, and it would invite their customer base to compare brands. (Cool! A new Nikon mirrorless...... But oh wait... I would need to buy new lenses for it... If I have to buy new lenses anyway... maybe I should go look at the Sony mirrorless...)
A key factor is that those traditional dSLR lenses did not play nicely with mirrorless. So going mirrorless would mean a whole new lens system, and if you start a new lens system from scratch, you are inviting your longtime loyal customers to switch brands. The EOS M system initially had terrible AF with all lenses, but even now, I understand it is only decent when using STM lenses.
But the A7rii may be the beacon of light -- If it can really drive traditionally lenses effectively... We need to wait and see... Then On-sensor PDAF can eventually drive all traditional lenses. Which would mean that Canon and/or Nikon could try to copy the technology (within the limitations of patents, etc)... and adopt a mirrorless system that preserves their old mount compatibility. If they can do a mirrorless system with FULL lens compatibility, it would remove their biggest reason for avoiding mirrorless. Of course, if they truly keep total lens compatibility, then the camera will be larger than physically necessary.... So they still need to consider -- Keep mount/large mirrorless... or new mount/more compact system.... or new mount/more compact system/with deeply integrated adapter for traditional lenses.
