Wouldn't it be grand if it were only this simple?
Lower level players stay there for a variety of reasons. Not aiming straight is way down on the list. Particularly for 3s and 4s. They have no stance, they have no stroke, they have no pre shot routine, they have no plan. In short they have no solid fundamentals. And then they often have booze and dope to boot.
And they generally have no inclination to change any of that. The reason the people you help improve rapidly is partly because of what you show them about their eyes but mostly because they do have an inclination to get better which is why you met them to begin with.
Great post. Fixing an eye dominance issue won't help a whole lot without solid fundamentals and lots of practice, which appear to be the main things lacking for most low-skilled players. For those who actually want to work on improving their game, seeing the shots becomes important.