I think the top pros do improve. But, the areas where their game improve is more in their heads than in their arms. Their improvement comes more in their mentality and focus from the experience they gain tournament to tournament, and match to match. The mental growth is then gonna carry over to their performance. Even when it appears their physical ability has seemed to plateau or deteriorate with age, their knowledge of the game will always increase. A couple of good examples would be Jayson Shaw and Scott Frost.
Jayson when he first came to America was an extremely talented shot maker, with other decent skills. But, as he gained experience and his knowledge increased, he then became able to gain the maximum potential of his already existing physical ability. Now Shaw is one of the most feared players in a tournament that he enters.
Now Scott on the other hand, was, and is, one of the best one pocket players in the world. His nine ball abilities on a pro level were considered mid-level pro. But, in the last year he has set his mind to wanting to earn a spot on the Mosconi Cup team. He has devoted much of his time to playing more nine ball and less one pocket. What has the result been? He has won the Rum-Runner, which is one of the toughest bar table tournaments and one that SVB participated in, and is in the top tier of the Mosconi Cup standings. But the most improvement in his abilities is up stairs in his head which allows to execute his physical abilities at a higher level. His physical abilities in nine ball has increased, but it first had to come from his mind and his determination.
The mind is where players will always improve, unless they develop alzheimers or something to that effect, even after their physical abilities begin to diminish. For a lot of players it's just a matter of honing of their mind while while keeping the confidence in their physical abilities.