I think everyone reaches a peak where the balance between physical ability, mental ability, practice time, and money comes into play.
There are other peaks that no matter how hard you try, how much you practice, how much equipment you buy, how much coaching you get, you just are not going to see improvement.
Even as your physical game peaks you continue to hone your mental game.
Some of the decline is do to our society’s short attention span. Some will look at it like a video game. Something they can master and then go to the next one.
Others reach a point in the game where they have gotten as good as they want to and then move on to a new challenge.