That's an interesting question, why do we feel the need to continually upgrade our cameras?
No doubt there are different reasons for different people. If one can always easily afford it, then having the 'latest and greatest' might simply be ego-boosting, like 'keeping up with the Jones's'. Any practical advantage might be secondary.
Speaking for myself, my main motivation for any upgrade is based mainly on image quality improvement, weight and ease of use.
Since I always shoot in RAW mode and process my images in both ACR and Photoshop on a high resolution monitor, I easily become aware of any technical deficiencies in my images.
It is as a result of such awareness that one might question how such deficiencies could have been avoided, if one had used a more sophisticated camera.
Perhaps the ruined (or less than ideal) shot was due to misfocusing. An upgraded camera with more accurate focusing might have avoided the problem.
Perhaps the dissatisfaction with a particular image, is due to noisy shadows which contain interesting detail which one doesn't want to blacken. An upgraded camera with better DR might have avoided the problem.
Perhaps the dissatisfaction is due to the inadequate resolution because one didn't have the ideal focal length at the time of the shot, and one has to crop the image heavily. An upgraded camera with a sensor of significantly higher megapixel count, might have enabled one to make a sharp A4 size print of the heavily cropped image.
These are the sorts of considerations that influence me when I make a decision to upgrade.
One should also bear in mind that the basic resolution of an image is dependent on both the lens quality and the sensor megapixel count. Increasing or improving either one will improve image quality. A Nikon D850 image downsized to the same 12 mp size of the D700 will always be sharper than the D700 image, when using the same lens with both cameras, and appropriate shutter speed and focusing accuracy.
An upgraded, higher megapixel count sensor, effectively upgrades all of one's lens. That could be a real bargain.
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