From what I have seen over 30 years of training, many people reach a certain level - doing the techniques pretty well - and then they just stay there. And many slowly get worse over time due to not training hard or often enough..
Yes, that is the issue behind The Legend of Bagger Vance. Ranulph Junuh is a great golfer who has lost his swing and Bagger Vance is a mysterious caddy, who appears and helps him find it. What you're describing is really the fate of many Aikidoka, who master aspects of the game (particularly the later aspects), but never reach a level of complete mastery.
That is probably also why teaching offers a burst of growth in the beginning, because you have gone back and worked on your basics, which you missed as you rushed impatiently toward Black Belt. So all that kata training is like spending time on the driving range, putting, chipping, etc. There is not a second way to drive a ball off the tee. And you can't be a truly great golfer unless you have perfected
your swing, your putt, your chip, etc. no matter how good your grasp of the game is.
Wringing out that golf simile, if you first "master the strokes", then the knowledge of the rules and tactics, then the wisdom of play, THEN you will avoid the inevitable plateau and staleness, because you can proceed to the 4th level of true mastery (transcendent love) instead of going back to correct the elements you missed on the way up.
I'm not sure if it was by design or accident, but this seems to be the path O Sensei took. Banging out Daito Ryu and all those other martial arts for the first 40 years, then spending the next 20 years working out the intelligence and wisdom of it's application, he did finally transcend it and arrived at a place of peace and love. But you can't just pick up where he left off. You have to get there first.