Yes, I see how this is could be the main usage scenario. In a context of a single roll everything above makes sense. Yet, single frame scenarios are I believe are no less important, whether it is sheet film or individual slides.
Technically that would work, realistically not sure. It is hard enough to match a given color with sliders if the example is already on the screen. Matching a color on screen to its real world idea would be even more difficult. Of course, an approximation would work and it could be a useful tool, but how different would it be from adjusting the ratios?
Six sliders would not be very user friendly. Darktable’s solution where the overall balance is adjusted by the RGB sliders for me is hard to use. The reason is in a real scene the dominant illumination color is rarely R or G or B, it is more often somewhere close to the planckian locus you mentioned, which in RGB terms is a mix. Maybe HSV would be more convenient though. Or maybe a freehand choice from a color wheel would be even easier.
The scenes I mentioned before may well contain neutral gray spots, but they lit by colored light, so nothing in the scene is neutral anymore. The difficult thing is to reproduce the color of that light.
Anyway, this is just one opinion. Maybe @Ilya_Palopezhentsev has a different one.