The problem is that those cameras will not have the latest sensors. I wish I could get a DSLR with the sensor of the R3. And IBIS in a DLSR would also be nice, but I understand that this is not possible without increasing the flange distance.
That is true, but at the moment I use the 1D X, that came out in 2012. Back the cameras were already so close to perfect that you could basically use them forever. At least the flagship cameras. The only real downside I see is the really bad noise performance when it comes to shadow recovery. Even at ISO 100 heavy noise is introduced if you raise the shadows. Apart from that it is hard to see that those photos were taken with such an old camera.
Most of the "progress" the cameras made in the last decade is stuff that I do not really need. For example I do not need video at all, as skyscrapers normally do not move. I do not need any better autofocus than the one I already have, as it is almost impossible to not get a skyscraper in focus. I do not need a faster burst, as the skyscraper would look the same in every frame. I do not need eye detection, as skyscrapers usually do not have eyes. I do not need GPS. If I take a photo of a skyscraper, the geotag should show the location of the skyscraper, but with GPS it shows my location.
I hope I will still live many decades, but I have no idea how cameras could be improved much further from here one. The only additional feature I would love to see is the option to program the camera to take a few photos with different settings in quick succession with one click. For example always one version with low ISO and one with high shutter speed plus one with a wide aperture that allows me to clean dust spots in post more easily. That would save me so much time.