I don't train for belts, but I do like the idea of grading. In genbukan, all kyu levels are green, all dan levels black. The idea of grading is nice because genbukan has a fixed curriculum for each grade, and grading is the same across the entire org. Reaching black belt takes 7-10 years (if you train hard enough) so the emphasis is on mastery of the curriculum, not a succession of belt colors.
That said, koryu systems still work without belts or grades (well, kinda) so I guess both approaches work.
While I don't train in any of the kans, I do like what Mr. Tanemura and Manaka, have each, respectively done with the ranking in their systems. While you do wear a green belt, you are still moving to various kyus, so while the belt doesnt change, the rank technically does, even though again, its the same belt color.
Your last paragraph makes alot of sense, and IMO, is what alot of people seem to miss. The emphasis is on mastery of the material, not the belts. Now, given that you said it takes 7-10 yrs to reach black, with alot of dedication, I wonder...do these arts teach kids? I ask this because kids seem to be the bread and butter of alot of schools, thus, why you probably see a bunch of 8yr olds running around with black belts and 12yr olds with 2nd and 3rd degrees. If kids are taught in the Genbukan and Jinenkan, am I safe to assume that its re-enforced that they will need to put in the work and they will not be handed their belts, they'll have to earn it?