Sure, that can help.
But my point is that if a player doesn't have "it" after 5 years, then no amount of lessons is going to give him/her "it"
There's no such thing as the "pool gene". No one is born specifically to play this game, but there are a series of genes that favor games and sports that require spatial awareness, pattern recognition, and hand/eye coordination.
I'm sure there's some truth to what you say about genes, but I don't think it's accurate to categorize someone as genetically unfit if they don't improve to the level they want after 5 years. In fact, maybe the OP's got all the right genes and his problem lies elsewhere. You don't really know what his issues are.
There are many variables that can be changed and can influence improvement, such as attitude.
For example: A player, while practicing, misses the same shot 3 times in a row and then becomes negative and discouraged. He does that repeatedly without realizing that he has been programming his sub-conscious mind to trigger a negative reaction when that happens, and as a result, gives up too quickly.
But with a little introspection, that player can be more aware and uncover these sub-conscious triggers, and then can start to eliminate them and improve his overall attitude towards the learning process.
That's just one example of a whole list of variables that can be changed in order to improve performance. An instructor who knows how to recognize patterns like this, can really help a player who has been stuck, to significantly improve. I know because I've done it with clients, and it works.
Each person you help is a puzzle that needs to be figured out, and no two people have exactly the same issues.