I like you last paragraph and agree with it in large. But, how do you explain the exceptions? People with incredible physical or mental ability such as inventive scholars and Olympic level athletes? Or on the other end of the spectrum, people with no known deficits who just cannot figure out how to effectively perform the most basic techniques?
Leave out the extremes, your next set of statements will give you a clue to my thinking.
I have a hard time putting people in two categories (yes, I get the male/female reference). We are creatures that are all built a little differently. The "art" in learning and especially teaching a MA technique is divining these inherent differences in people and showing them how to make the standard model of the move work for them.
My philosophy is that the standard model, traditional karate which by my definition includes TKD, TSD, works broadly for everyone, all of us. As a general rule, those tinkering with the model or creating "new" systems aren't adding or creating anything which hasn't been done over the history of traditional karate, and then traditional martial arts going back to China or other eastern areas.
It is not an obvious thing most of the time. Maybe after hours and hours, usually years and years of doing it.
And here is the divide between me and others. When I really started training traditional martial arts (adult), I came into them with the express goal of unraveling and understanding the principles underlying them, behind the skills & effects.
I was fortunate and lucked out in that my first TMA instructor was an intelligent person. He had more or less taken the same approach in your last quote block, himself, and myself. He didn't explain in a lot of descriptive detail like I do, however, he was very precise in both the design of the curriculum and in his instruction.
Although his original TMA was TKD, with the school's curriculum and training regimen along with very precise instruction (not hand holding), I very soon came to realize how for starter's TKD and the Japanese karates all had the same underlying basis, all had common principles.
I absorbed the excellent curriculum and progressed very rapidly. So much so that in months, I defeated two of the three assistant instructors in sparring. That was no-contact sparring. In that short time period though I was in shape physically, I hadn't really developed any karate power, the whole body strength and then beyond.
I never sparred the head instructor because one, my training roughly paralleled the manner in which he trained... he really didn't spar much. We had come to the same conclusion that training the curriculum to it's principles develops the true & high level abilities and skills. We never sparred because we both understood it to be a waste of time. I will never catch up to him in ability for a host of reasons. For practical purposes, I knew I would lose.
We had another student there at the time I joined. He was Mr. Boxer, fighter, what have you. He used to constantly challenge the head instructor. It was truly a silly display of ineptitude on the challenger's part.
SO the divergence between me ,myself, and all those who want to improve TMA or traditional karate with some add ones, significant modifications, extensive cross training; PLUS, want to include a goodly measure of spirited sparring with actively resisting, physically forceful non-compliant opponents -- my approach was to focus internally, practicing the curriculum of kihon, kata, kumite as tradition called for. With sparring supplemental. Sparring for testing.
Now these two of three assistant instructor which I defeated within months of (concentrated) training, of course full contact would have been a different story. Their strength base was above to considerably above mine. My goal was to re-devote my energies into the curriculum in order to close the base strength gap.
And that's what I did and that's what I've always done. And that is the reason I've defeated every karate instructor I've ever fought. Part reason being they weren't as good as they thought by training the way I read so many here. And probably some more emphasis on the active sparring and resistance testing would have helped them... as so many here propose. But not enough to make up for where I went.
I don't go around challenging karate instructors either. I don't challenge superior skill. That's dumb. It's a waste of time & training effort when (if) you realize the curriculum provides the answers to defeating strong opponents, not fighting them. This IMO, is the traditional karate model in its pure form.
BOTTOM LINE: My philosophy is that its' the curriculum laid out by the masters which provides the answers to martial success. The base for that. The weakness in TMA practitioner, is their weakness in not truly understanding what the TMA model is.
So they try to make up for that by doing a lot of sparring. Making "improvements."
Is learning the traditional karate model as you put it difficult? YES. Is understanding it problematic? SURE. Is it easy to get sidetracked by other styles or trying to make the model be what you want or think it should be? MOST definitely. the masters weren't gods or all knowing. But they did their homework. They knew, basically, what constituted a successful model. It's the rest of us who must live up them, the standards they set. That's the challenge of TMA.
Based on that conclusion, then the premise for why TMA is under-represented and not so successful in the commercial MMA forum is (1) TMA is harder to master to a solid working level (incorporating the lengthy time investment you speak of), (2) one has to realize how to best approach it, in order to tap it's true power. This is a minority.