I've been doing a lot of reading (and posting) of late and this thread (and the questions it raises) are a favorite. I mean, what could be more exciting than the possibility of replacing your entire setup with something smaller and cheaper and yet more powerful. But I'm not seeing it.
I'm still new to the idea of mirrorless, but it seems less of a problem looking for an answer, than the other way around. Starting with a bit of context, there are two things going on with the camera industry (I'd also say the death of film, but it's already so complete..): 1) point/shoot cameras are being swallowed up by the cell phone and 2) companies other than Nikon and Canon are watching for something disruptive to knock these established brands off their pedestals.
The answer that is mirror less, is trying to do both. Fuji and Sony and Panny have rushed to market with all new systems, catching Nikon and Canon off guard with something radical. They are also trying to inspire budding phone photographers into making dedicated camera purchases. Canon stumbled in with the M and appears poised to leap ahead with dual pixel focusing on the next model. But it's still not clear where we are going. It's like we are missing the pied piper (or Steve Jobs) to show us the way.
Does mirrorless offer SLR-like lenses to people who have outgrown App Store options, yes. Does mirrorless offer SLR flexibility and control to people who don't need the full ecosystem of SLR options, maybe. Does mirrorless offer environmental connectedness of a real through the lens viewfinder, no. So mirrorless has done better at only one mission, but it's not a bad approach for a lot of uses, either.
A major advantage of crop sensor SLRs is that equivalent length lenses are shorter/skinnier/lighter, because the target square is smaller. Mirrorless takes this a step further by moving the sensor closer to the glass, now equivalent length lenses can be even shorter/skinnier/lighter. But I can't write that sentence without the word 'length' because they are not fully equivalent. EF lenses come in 3+ classes, and as long as people buying the top (L) aren't getting an M instead of a 1D or 5D, people wanting that glass must do the same. It's the main thing that will make someone buy a 5D4 instead of a 7D2. And it will be the main thing holding back a truly professional M.
The point of an SLR is to have a high quality viewfinder. As long as that's important and as long as that is not available on mirrorless, the preeminence of SLRs will continue.
if that's the case they better start turning a profit because without Nikon and Canon the mirrorless market is doomed. Nikon and Canon are the market. they are the ones making money on their cameras.