I think the reason it seems like these skills take 'forever' to learn is that most of us don't spend our full work time on them, as Ikeda Sensei has. If I practiced aikido 40+ hours a week, I'd be much better in a hurry! I think Mike is absolutely right - the most exotic technique or blending can be mastered by anybody who puts in the effort.
I have been learning from Ikeda Sensei for 15+ years now. Whenever I have the chance to take ukemi for him I attack hard with the thought, 'you're not getting me so easily this time, buddy,' in my head. Of course, he always does. I find I learn the most from him when I attack with the most sincerity.
And now, off to the dojo.....
Sure... to be Shihan level, it takes every day practice for years. But to understand what is going on and do it in the controlled environment of practice, it should not take so long. The teaching methodology just hasn't been there.
Some of it is language barrier. Ikeda Sensei will say "make tight" but he isn't tight at all. He'll say "pull" and he isn't pulling, and "push" and he isn't pushing... not in terms of the set of associations we have in our bodies with those terms.
One of my friends who actually speaks Japanese and is advanced enough to have something of a handle on what is going on, told me that he got to hear Ikeda Sensei explaining what he was doing in Japanese to a Japanese woman attending the seminar. He told me that Ikeda Sensei's explanation was so much more clear and precise in Japanese than it was in English.
I understand most of what Ikeda Sense is doing. He will point out three steps in what he is doing. He is trying to be helpful. But often, I happen to know that inside those three steps are several other elements which are crucial elements in accomplishing what he just did. If you don't know that, you could be doing exactly what Ikeda Sensei had told you to do and still be wondering why your stuff wasn't working.
If you train with someone like Mike S or Dan H, just as an example, the level of detail in the explanation is so far and away more complete compared to what we routinely have gotten in Aikido that one starts to wonder how anyone actually got any good at all training the way we have.
Only two things are required for pretty much everyone to be able to do what our teachers are doing. First, is teachers who understand what the "big guys" are doing. Mostly we don't have that. Second, they have to be able to explain it to others. Unfortunately that is a smaller subset still.
Training the way most of us have trained over the years and hoping to end up with the skills these folks have would be like the fifty million monkeys typing Shakespeare... might actually happen, but the operative concept is that the other four hundred ninety ninety million were typing gibberish. With decent explanation virtually any student on the mat at one of Ikeda Sensei's seminars could do any single thing he showed all weekend. With a real newbie it might take a half hour to get him to do a given technique or exercise and he certainly wouldn't be able to generalize off that success to apply the principle in other contexts. That does take years of practice. But what these teachers are doing is not magic and it can be taught and explained to pretty much anyone.