Learning the physical aspects of the fighting arts does not take much. One who is semi-athletic can learn all the required physical aspects of the fighting arts in a short period of time. One can actually earn a black belt level leaning only the physical.
You might say, not in my system! Well, I am speaking strictly from the point of view in regards to how the Marines earned Sho-dan back in the day on Okinawa. Here is what I believe happened. There is no proof and if you ask the first generation practitioners of Tatsuo Sensei they will say I am way off-base. Anyway...
In the beginning there were only Marines (a few other services of course but for this narrative it was the Marines). Marines, on the whole, server about thirteen months of duty on Okinawa. Yes, there were exceptions, there are always exceptions.
Marines fell in love with Isshinryu and thus spurred the contracts to teach through the special services system. This was a boon for Tatsuo Sensei and his family especially since those were hard times, those early years, i.e. fifties and before. Marines like titles and the black belt was a symbol that the Marines could get their teeth into so the goal was to get that black belt. This is not much different than today's practitioners where getting a black belt seems to be the "goal" of training and practice (lets not forget the trophies, titles, etc.).
Tatsuo Sensei arranged his structure to provide a means of awarding black belts, i.e. sho-dan. He understood the desires of the Marines who attended the Honbu dojo and that making the Marines happy and satisfied he needed to award the level yet he also felt it important that the award was warranted.
In some places learning the basics and one to three kata can qualify you for sho-dan. This would include bunkai, the gokui, and may other more esoteric learning vs. strictly the physical. Tatsuo Sensei understood this and a lot more so to allow a one year turn around for black belt he focused first on the physical and for those who chose to go beyond the mere physical to levels higher than sho-dan he provided the Kenpo Gokui and many of his quotes in training. Those provided a key to another door to go beyond the mere physical.
Few chose this path simply because it meant leaving the glorious physical in the back seat.
Only a few really chose to go beyond that which strokes the ego, i.e. higher levels and that stuff that didn't involve belts, trophies, certificates, and the accolades not to forget titles such as "master" or "Kyoshi", "Hanshi", etc. which really feed the ego more than represent any "real" accomplishments.
(CAVEAT: I realize this statement may offend and some might misunderstand that this is close minded or they will think I am full of s%^# but understand this is just my observation from my viewpoint which is not worldly, etc.)
We also forget that Tatsuo Sensei understood better than any of us did at that time and even still today that earning sho-dan, first level black belt, was not this significant milestone but just a way for the Sensei to tell a person that they have gone the distance to be considered, finally, as a serious student. Sho-dan is the first level of being a real "deshi!" and is only the very beginning. All the rest before was creating a solid foundation to work from in real training and practice.
The Marines got the physical which was important for the Warrior in them and they got the coveted black belt. They also got the keys to the next level but many forgot them in the rush to return home, open a dojo, and gather as many adoring students as they could to pay the rent, etc.
The few who got the keys and actually opened the door are few and far between. Those are the true descendants of the Tatsuo Sensei lineage. Much like the Marines, i.e. "The FEW, The PROUD, The MARINES!" for there are only a "FEW," "PROUD," and "TRUE" descendants of the Isshinryu that Tatsuo Sensei built.