You teach them to fly and don't expect anything. Some will get bit by the bug others will get bored and move on. You can't control what people will do in the future other than giving good instruction that is fun and then let them decide if they will stay with it. I don't understand the sentiment of some about the need to give back or encourage people to stay or any other seemingly altruistic motives. I teach because I enjoy it, it makes others happy and frankly I like the adoration from the student that comes with being looked at as an instructor that helped them. Other than that and the good karma from teaching I don't expect to get anything out of it and I don't get upset if they move on to something else.

If you look at the amount of people that started as kids and then came back to the hobby later in life I think you'd feel better about them leaving the hobby. Many return but on the other hand my field is pretty crowded allready.