I maintain a Phase One H25 from 2003, processed in Capture One, produces consistently richer color, better detailed, smoother transitions, and more pleasing grain than today's Canon 5D3. It has all sorts of limitations (no LCD, no long exposures, ISO above 100 is awful, no on-board storage, slow frame-rate etc etc) but the quality of color and tonality in medium format has been excellent for much more than a decade.
In my opinion the heritage of medium format image quality has been successfully passed down across the years such that the current models still have that richness of color. In fact the same gentlemen (Niels the "Image Professor") who made the color profiles for your P21+ is the one who
made the color profiles for the IQ250.
But only you can be the judge for yourself. I'd be glad to send you some raws (email me) from more modern digital backs like the P65+, IQ140, IQ250 etc and you can tell us what you see in comparison to your P21+.
As for other types of cameras, my first thought is always that people can become overly connected to one part of a system and forget that you don't create a photo with a sensor, or a lens, or a piece of software - you create a photo with an entire system. The look of any given image is always primarily a result of the lighting/subject/composition, and secondarily from the lens/sensor/software.
The most interesting component of this, to me, is how the camera part of the equation often influences the lighting/subject/composition part of the equation. Even if logically there is no reason why you can't shoot exactly the same way with two different systems, in reality many report that their shooting style is influenced moderately or significantly by their equipment - human beings are not creates of pure logic.
