I think CMOS still, digital-camera performance has plateaued.
Most recent improvements involve niche applications instead of sensor advances.
For instance, 40 MP (uncropped) allows a 24 x 35" print with almost 300 dpi. This means lower detailed subject matter could print at twice that size. A fine-art photographer could make money selling larger high-quality prints. Some commercial corporate photographers could benefit as well.
As Bill mentioned, higher performance in-camera stabilization is another example.
One niche where vendors could make a significant improvement in raw file technical quality is true monochrome raw files. Monochrome versions of the Nikon D5, Panasonic K-1 and several SONY 24 X 36mm sensor cameras could gain about a stop of low-light performance.
Since I am no longer doing gigs, what I care about most in a digital camera is having an OVF, raw file technical quality. and lenses.
Very few digital cameras have OVFs.
Raw file technical quality has two aspects. [1]
One is the signal-to-noise ratio at a camera's native ISO setting. This is confounded by dual conversion-gain sensor technology where a sensor has two native ISO settings - a lower setting when dynamic range is a priority and a higher setting low-light performance is a priority. The former maximizes signal while the later minimizes electronic noise.
The second aspect involves the sensor cover glass, IR filter characteristics, the color array filter properties and the micro-lens array optics.
For my needs practically all digital cameras marketed in the past 3 years or so have excellent raw file technical quality.
Lens performance has plateaued as well. On-board lens correction parameters provide great cost-performance benefit. Repurposing older lenses adapted lenses is an inexhaustible source of exploration.
My motivation to purchase a new digital camera is non-existent. Spending money in Bill's hypothetical camera store that camera store sells new, improved working skills would be much more productive.
More practically, spending money on prints is much more attractive than new cameras or lenses. Outsourcing professional scans of carefully selected negatives and transparencies is another option for spending money on personal photography.
1/ In digital imaging the information we use to render images is incorporated in the signal. The noise determines the signals' uncertainty. More SNR means more information which in turn enables higher perceived image rendering quality. Increasing SNR is the only way to increase perceived image quality. Noise filtering (a.k.a. noise reduction) can not increase an image's total information content. Noise filtering cleverly averages information in higher SNRs regions with lower SNR regions. The result is not an increase in total image information. The result is an improvement in our perception of low SNR region rendering at the expense of high SNR region information content. Fortunately, this compromise is compatible with how we perceive IQ.
