The biggest problem for mirrorless is that Canikon are deeply entrenched in the pro scene with pro support and the AF tech. and the f2.8 pro lenses are a god send for many since the bokeh is effectively like a Fujifilm/M43 prime throughout the focal length. I doubt Fujifilm are going to make a 16-47mm F1.9 for it would be too big and heavy for their cameras.
they got speed AND low light performance while Fujifilm have the low light and M43 have the speed.
They don't need FF sensors, they just need to improve the rest of their tech, get those pro lenses out and have pro support, the rumoured 24mp X-PRO2 would help those who want to print larger or need the resolving power.
But even though Fuji (actually APS generally - nothing particularly notable about Fuji) has good low light performance and M43 have good speed, neither is up to the standard of full frame DSLRs in those areas. APS is pretty good in low light, phenominal by the standards of just a few years ago. But full frame is notably better - the A7 and A7S on the mirrorless side and various Nikons and (I think, but I'm less familair with) Canons. And the dynamic range (and associated processing latitude) of the current full frame sensors in the A7, A7R, D610, D810, etc is something you have to work with to believe. Again, APS is good but full frame is in a different league. And for speed, mirrorless has burst rates faster than DSLRs (since they don't have the physical impediment of the mirror) but they still can't track AF at those speeds anywhere near as well. They're getting closer - the GH4 seems to be on the right track - and they'll certainly get there and surpass DSLRs, but they're not there yet. To do really high end tracking AF at 11 fps, you need a top of the line and enormously expensive (and large) D4, D4s, or the comparable Canon, but you can track at 5-6 fps in the D610, the Df, 6D, etc... As well as in some of the better APS DSLRs.
Canon and Nikon will jump into mirrorless when they see a way to make more money at it than they lose from it. Once mirrorless cuts into their APS DSLR sales enough that they figure they're not really gonna cannibalize their own stuff much further, I suspect they'll jump in big time. I suspect that's not too many years off. IF Sony or someone else makes serious progress with full frame mirrorless, the same may happen with full frame, but I suspect that's a good bit further off before mirrorless can get close to the performance of pro-level DSLRs. Once there's a good full frame mirrorless system with a decent compliment of native lenses, I'll likely move back to mirrorless
IF THE OVERALL SYSTEM IS NOTABLY SMALLER - I have my doubts about how small the lenses will be even if the bodies get smaller and smaller. I don't really care much at all about burst rates and tracking AF personally. But for a pro level shooter who cares about all aspects of current DSLR performance, I think it'll be a while before mirrorless can compete with the highest end Canikons...
Mirrorless is in it's techological infancy and has a huge margin for improvement where DSLRs are pretty mature and are only likely to improve incrementally, sensors aside. No question mirrorless will overtake DSLRs in pretty much all performance measures in time, but I don't think it's gonna be all that fast because DSLRs today work sooooo well and have such an incredible range of available lenses. It's gonna take a while before mirrorless catches up in ALL of those areas...
-Ray