I'm a photo retoucher by trade. I bought both the iPad Air and the retina Mini last year, and saw the difference in color gamut immediately. But I quickly forgot all about it as soon as they were separated.
If someone who is paid to look at and think about color all day can overlook a slight narrowing of the screen gamut, then most people can't even see the difference. I'm betting Apple won't bother changing the screen for the next Mini.
Sadly, I'm betting you are right on the money in that Apple won't bother changing the screen on the next Mini because most people can't see the difference.
However, I wouldn't characterize the color gamut of the Retina min as "a slight narrowing of the screen gamut". A Displaymate shootout reported that the Retina Mini has only 63% color gamut (identical to the original mini....which i also owned). That's signigicant when compared to just about every other premium tablet (i.e. most of which are around 100%).
I'm wired to pick up nuances and I noticed the muted colors of the mini every time I used it. Every time I picked up my wife's Air, the difference in color reproduction and vibrancy between it and the R-Mini was shocking. If google/Asus and Amazon can manage to produce small tablet's (i.e. Nexus 7 and Fire HDX) with 100% (ish) color gamut, I'm sure Apple can do it. Will they do it is another question. I'm skeptical for some reason.
Here is a comparison (below) I did last October of the same photo displayed on the IPad Air (left) and the Retina and non-Retina mini's on the right. I forget now which is the Retina Mini and which is the non-retina mini. But, doesn't matter, as they both have the same 63% color gamut. BTW...the flowers were actually purple (as displayed on the Air), not Blue as displayed on the mini's.