I doubt that's why people upgrade. If you made the sensor replaceable, people would still be using digital cameras from 1996 except with the latest sensor.

I have several replaceable-sensor cameras and the bodies/lenses were made before I was born. They still take great pictures because it turns out that focusing light onto stuff is largely a solved problem. (They don't even use batteries, much less need an up-to-date OS! But I will admit that carrying a separate light meter is a little annoying.)

Yes, there will always be people that need a 1 billion point autofocus system, and that will require some heavy software engineering. That is a niche use case, even more niche than needing a digital camera to begin with.

Big cpu means shit battery time, and thus a shit camera. And locking into a specific hardware configuration in a field that is basically all about making hardware developments seems kind of short sighted.