Milliern: I'm with you here too. I suspect the limiting factor (for ratings less than master anyway) is motivation.
I might agree with that, but maybe not. I don't know. I think that any instance in which rating progress seems to disappear probably isn't an insurmountable "magical knowledge wall," but a indicative of the fact that 10 rating points, say, from 2050 to 2060, contains (i.e., requires) more knowledge than a jump from 1250-1260. If my friend, Abe, consistently made 25 point increases per year, then it becomes 10 points per year at 88 (supposing he makes 2000), it might not be age, but the fact that there is more information to learn and technique to develop in a ten-point span in the Expert division.
I agree with your point that each rung up the rating ladder requires mastering more information than the same rating interval below it.
That would make motivation an important factor in continuing improvement, which was my point.
No, I would say that it rather is more information available for lower rated people in a way it is easier to improve, while stronger players don't have much to improve on.