When sony or whoever else comes up with a aps-c sensor that crams 24mp into a small area, could they not scale that same surface area to a 645 size sensor ?
They probably could, but most likely are not interested in the dwindling market for various reasons.
The main thing is that the principal CMOS sensor makers make cameras and know what can be achieved with the formats they are already working with.
The very top end of CMOS cameras is so close today to what MF can obtain that they see no growth or stable market.
I'm sure that Sony is looking closely at what Zeiss is doing with the new line of DSLR lenses. Sony also has a good idea of what is going on in the MF world.
Just look at Hasselblad's desperate move to make money with the Lunar.
Sony would only make the required investments if they could get a guaranteed order quantity and if the company ordering were solid enough for the business
to not be too risky.
Regarding Sony, Canon or Nikon coming out with a medium format camera... that is unlikely.
One of the reaons is that the Spin type marketing required to justify MF to more than the users that really do need it is not in their style.
I think that if a new entry were to be made it would have to be a larger than 645 format to really be able to differentiate itself from 35mm DSLR.
While MF manufacturers like to talk about a sensor twice the size it actually is a sensor that is twice the area, however resolution and most visual characteristics are more of a linear function
so the size difference really is not that significant.
Sensor size difference between a 35mm ff DSLR and the biggest MF sensor is not that big if you look at it from a linear point of view that is more relevant.
36 vs 53.7 is a 49.167 % increase.
To summerize Sony, Canon, Nikon etc would simply look at these two images and then tell themselves why bother and why take the risk.
both are crops from this framing:
One is 35mm DSLR and one is MF digital.
The CMOS manufacturers are way more likely to focus on improving their camreas and sensor to compete against their more relevant
MF digital is really a sector for those companies that are stuck in that format and their per item margins have to grow in order to survive in a dwindling market.
Five or six years ago high end fashion photographers would be using MF nearly all the time, now most are using 35mm DSLRs