Bob,
I agree with you that after a certain amount of time we can't materially improve. Or to put it another way, we won't get much better at hitting a little white ball.
But I think that most of us don't play up to our potential. I think we can improve our mental game a great deal. I am playing about 3-4 times a year right now (last few years) but I shoot about the same scores that I did 15 years ago when I played 50-100 times a year. I'll agree my swing hasn't improved, but given the lack of play/practice, I should be so much worse. In a way I am much worse, as I don't hit the ball anywhere near as well as back then. But I'm way more likely to make smart decisions and play within myself now.
I think a similar improvement can be experienced by anyone. It may not be a material improvement, but it is a handicap improvement.