The reality with 46 mp on FF is that you are only gaining potential resolution via more pixels. The magnification and light gathering is still limited to the FF format size. It takes takes resolving power of the lens to use those extra pixel densities, and magnification on FF isn't that high.
This is why people are not seeing the benefit of real extra 'reach' on the crop sensors, or at least nothing like they would expect when comparing up sampled central 8 mp from the FF and 18 from the whole of the crop sensor.
In practice you don't just need more pixels for more resolution; you need magnification and light as well. The crop sensor gives you the extra pixels but no more magnification and light than the FF sensor cropped to APS.
In just the same way you can up sample the image without losing anything like you would expect so you can do the same with a 23 mp FF sensor against a 36 or 46 or whatever FF sensor. You can also run into the issues of smaller pixels receiving less individual light and less saturation etc.
I'm sure the reason why Canon haven't yet introduced a 'very high' mp FF sensor is that in reality there just isn't much point to it.
