If you take money/business aspects out of the belt question and focus on learning, belts are not at all that important. They are just artificial grids superimposed on an individual's skill continuum. They are a convenient tool/frame of reference to help describe a skill level within a particular school or organization. This can be helpful as a motivator, organizer of info, and an indicator of relative ability amongst the instructors and students. It can make things easier for all concerned, especially in group classes where members/teachers aren't so familiar with each other. Rank is a mere description, not the thing it's describing.
For serious, self-motivated students who have a close relationship with their instructor belts are not needed at all. This described most all karate students prior to the 1920's, before it went public. Not by coincidence, there was little stress on belts/rank at that time. (Earlier for judo, later for Okinawan karate.)
The Orient is big on hierarchy, much like the military. Rank is very important in the military. But if you look at our Special Forces, rank plays a lesser role, small tactical teams more informal about rank. These elite groups fit the description of "serious, self-motivated." This description does not fit most modern MA students - thus the reliance on ranks.
Where does the relatively more "rankless" Muy Thai and some FMA fit in? Maybe three considerations: I think it's safe to say these two societies are less structured than Japan/China and without the historical military traditions. This perhaps made them less rank conscious. Secondly, their teaching was not institutionalized in the school system on a large scale like karate and judo were (I don't think). Thirdly, MT and FMA students are a smaller and perhaps more serious esoteric group, resembling the early karate students and not so rank conscious. But with growing popularity in the West, it would not be surprising to see a belt/rank system evolve in the future.