The current K1 36MP still holds its own against the D850 and the a7xxx in terms of image quality, color rendering, dynamic range, etc. You also need to consider the image resolution of the current lenses - being able to support 60MP. 36 is sufficient for most, 50 in my opinion would be tops - and that would be based on their current planning for a new 645, which would need to be 100MP. With Nikon's recent announcement of their new imaging sensors, I would think that some of Sony's private sensors would become available to the smaller lines such as Pentax.
Additionally, not everything is MP. With substantially larger amounts of pixels, you need the thruput performance of the image processor to handle the larger images, which would require additional battery power. Currently Pentax has (even with the thruput of the K3III) a bottleneck in their image processing at 50MP and greater. Their current battery also presents some limitations.
They just updated their image processor for the K3III, and will probably need to roll it again to support both a new full and medium frame offerings.