The high pixel sensor will cause serious problems on the optical side. For example, if you go to DXOMark, there is nearly no lens able to match the 36 MP Sensor of a Nikon D810. The few who can resolve at least about 30 MP, are expensive primes... and even the Otus can't fully get the 36 MP on the RAW.
The most used Zooms like the 24-70 and 70-200 are between 20-27 MPixel, so if you really "need" 50 MPixel you should also change your daily workflow. For landscape shooters primes are mostly the way to go, but for the rest there is no way to take advantage of the sensor in any way. But you probably get the backdraws like 80MB RAWs and a slow fps-rate.
Thats also the reason why the Sigma Merril cameras are resolving details like a Nikon D810 (sometimes more, sometimes less). They just need lenses for 15 MPixel to get details around 30-33 MPixel (45 MPixel as claimed are far too much in my opinion). I recently read some optical document about the limitations of optical systems and there is a sweet spot around 30-32 MPixel for fullframe... of course you can get workarounds on this as the PhaseOne/HasselBlad Back are already doing it. They shift the sensor for half Pixels and interpolate the results. Maybe Canon introduces something like that, the shifting of the Bayer-Layer (without the groundlayer) was also a recent patent.
For me, the pixelcount is nothing to be concerned about, I like the way the Sony A7S "thinks". If I need more resolution I stitch together with longer focallenghts, but I know this is just for wider angles and without movements. I'm really interested in the next Canonproducts. For the rumours about a new EF-Line... I think there is a mirrorless fullframe Line coming, adaptable to standard-EF with an adapter.