It really depends on the application.
Which is why I said niche applications will see some benefits.
The progress made .... with AF
A fine example of niche really. AF on pro bodies hasn't been poor since film days. Sports photographers were taking great photos back with the EOS 1 on film, that technology has just evolved steadily and some has trickled down to lower end bodies.
The improvements have only been minor in each iteration and rarely a 'compelling upgrade'
the ISO64 images of the D810 have a look that I have never seen in my D800 files,...
Is this really such a dramatic improvement that non-experts would notice a difference ?
There have been some serious improvements over the last 15 years.
A big jump in resolution. Even entry level cameras will now deliver files that can be printed to A2 without problems and few people can display images much bigger.
Sensor sensitivity is now fantastic and can shoot in situations that would have been regarded as impossible 20 years ago, but do we really need any more ?
Self cleaning sensors make a huge difference.
RAW capture brought huge amounts of control.
What more do people need ? What could be a universal 'MUST upgrade' ?