Dave de Vos
I see this happening in go too. Shodan is a rank where players are more likely to "retire". 4 dan is too. Perhaps 4 dan in go is a rank where one realizes something discouraging: even though one has come a long way, the road ahead is much longer and much less accessible than expected.
I think this is precisely the same in martial arts and Aikido in particular. The way Sensei always presented the Idea of Dan ranking, Shodan meant you had paid your dues and now knew enough for the serious people to talk to. In other words you had worked hard enough on your basic foundation that real training could begin. A serious beginner.
Of course, the myth was that Black Belt meant you were some sort of expert. Many folks set that as a goal for their training... When they accomplished that goal, they moved on to other things.
Fourth Dan was always presented as the limit for the amateur practitioner. You might have a guy who had trained for many years but he'd top out at 4th Dan if he wasn't teaching. Ranks above 4th Dan were considered teaching ranks. So you'd actually lose people after all those years because they would drift away when they realized that they'd never go any higher without more commitment and effort.
5th Dan meant you were on the Shihan track. You wouldn't get a 5th Dan if Sensei didn't expect to move you to 6th Dan eventually. 6th Dan was a Shihan and meant you were a professional teacher.
Of course all that has changed now. The bar got raised because so many folks were getting 5th and 6th Dan that it simply didn't mean the same thing. So now Shihan is 7th Dan. People will get 5th and 6th Dan just because of time in grade.
Since we don't have competition, and one doesn't lose his grading just because his performance degrades in competition, as in Western fencing for instance, people after a certain point in their progression in rank simply plateau and stop trying to get better. They "retire" only in the sense that they no longer work hard enough to get better, and they devote themselves to dispensing whatever knowledge they gad previously acquired. That would be fine if they had been really excellent when they plateaued but more typically, they were simply mediocre. So they teach and don't progress and become the limiting factor in the development of their students. It might have been better for the art and those students if they had "retired" instead.
Now if the teaching issue weren't so intertwined with the time in grade and years in the art issue, none of it would matter. It wouldn't matter whether one quit or didn't, whether one was getting better or wasn't. As I said in another thread, if we had a teacher certification such as Menkyo Kaiden in the Koryu or some such that actually had nothing to do with the Dan ranks, and perhaps had re-certification requirements associated with it, then things would be a lot more clear and the training would be better, I believe. But that's not likely to happen.