Yes, so no manufacturer wants to make a APS-C camera noticeably superior to current models at ISO 6400. This would bring down the sales of full frame, and Canon wants exactly the opposite, forcing demanding amateurs to migrate to 6D (or more expensive) and L lenses.
I guess Pentax wants the same? Or Oly?
No manufacturer can
make such a camera. If they could, they would, and then they will make better FF cameras as well.
This was said many times. The hard limit at a given ISO (more precisely, at a given exposure) is the area. Same exposure (a.k.a. "same ISO ", which is not quite correct) over a larger area means less noise per unit area. As simple as that. Sensors of the same generation have some differences in how they record that noisy image but the the differences are too small to beat the sensor size factor.