Would a player who has been playing for years benefit from coaching/lessons? My answer is, “Yes” and “No.” I will start with my reasons for the “No” answer. The individual may not accept change and is looking for immediate results.
Some things are a given in tennis—the ball will eventually come over that net, and it is expected someone will return it. The challenge lies in that the ball does not always approach you in a friendly manner. Lessons assist in developing your hitting. Books and YouTube videos on tennis improvement are readily available, but a coach has skillful eyes to spot things that you are doing that may be preventing you from being a better player.
For those recreational players, tennis is about the social aspect. A lesson with a trained coach assists in proper ball placement (zones), proper court transition, and shot selection. Improving their performance assists them and their partner to perform smarter, allowing for an easier win. Keep in mind, all professional players have a coach. If they are capable of beating their coach, why were they hired?