In martial arts, in my limited experience, it seems to be the case, in the main, that it is the students with 'natural ability' that tend to fall by the wayside earlier.
Perhaps for as much as the first few years of training, they revel in their superiority because it comes easy to them. Then the 'work' begins where you are polishing and grinding out minute faults and honing tiny improvements. For the naturally talented, who are used to having things come quickly to them, this is anathema and I believe that many of those MAists seen as 'good' that fade away at black belt level fall into this category.
For the lesser gifted but more dedicated/determined students, the process of training rather than solely the results of it are of a greater emphasis than for the 'Aces'. Thus, on finding that after you get your first Dan, you can expect to train for a similar amount of time again before you reach the standard for your second, they are not fazed and carry on with their training.
So, in the long term, I think that you perceive a circumstance such as the OP posited i.e. that in the martial arts you see a path whereby it is persistence that pays off in terms of skill rather than any innate talent that you start with.
Of course, match born-with-it aptitude with diligence and you get an altogether different outcome .
I have seen the same things you have, and discussed it with others; you make a great point.