I agree with you that Scouts over 14 quit because programs can be boring – and Scouting is competing with other High School activities. If their friends quit – there is even less of a reason to stay with scouting.
I would like to make 2 points:
I don’t think that getting a new scout to 1st class in a year to 18 months is the principle purpose of scouting for this age group. I have always thought that the goal was for the scouts that are generally 14+ old to learn leadership by ensuring they train the younger scouts – and if the older scouts have developed an adequate training cu