Since the 50MP sensor will be only slightly lower in res than the 7DII equiv on FF do not expect miracles at ISo6400, just take a look at a Nikon D800 RAW at ISO6400... but what it will give you is the possibility to resize that file down to 20MP and it will clean up quite alot... even more if you do it in stages with some light NR between.
So do we need a 50MP camera... well some of us do, some of us don't. It is funny, we have a 1Dx doing 12fps, do we question it... DO WE NEED 12fps?
I only question the diffraction limit on such a small sensor with so many pickles? Yes the micro mirrors help but that is alot of pickles. I am sure however as a landscape or studio camera... with a T&S lens at lower ISOs it will be great!
Except that the sensor is 2.56 times the size of the 7D2 sensor.
The per pixel noise may not be much better than the 7D2, but the overall whole-image noise would be 1.3 stops better, just on sensor size alone, even if all they did was scale the 7D2 sensor up to full-frame size.