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. They teach but rarely train. It is very common. If they do well, they maintain what they have, though they may improve in other directions - like being a good teacher, or stretching, or whatever. If they didn't have high dan grade status (being in with the in crowd) to keep them up there they would have likely been ignored long ago. Students often can't spot this until they have been around awhile.
It is very hard to spot or admit in the self. Heck, might even be me!
There is a thing in sports science/ motor learning called blocked learning , that is doing things by repetition to get better. Others thing are random learning, stress training etc.... Different methods of learning have advantages and disadvantageous. Blocked learning develops skills to a point and quite rapidly too, continued blocked practice builds confidence however after a point the skills can actually go backward. Aikido training has many learning elements in its practice - which is great, but its easy to imagine that after 30 years it could all be just 'blocked learning' and so there little learning and even some decline.