BBDad,
No drop of in A or B schools because they wouldn't be able to form teams if any number of athletes dropped out. There is a lot of pressure in these schools to have nearly every person turn out (which I think is pretty cool).
As for hoops, I think that since overall athleticism, coupled with some genetic components (height) seem to be the main requirements of the game, I would agree that gross technical skills may not have improved over the past few years. I also don't think that hoops, or American Football, or Track and Field, have the number of single sport athletes you see in the Olympic Sports. I believe that those children who participate in the Olympic Sports (Gymnastics, Swimming, Ice Skating, Hockey, etc.) are under more pressure to specialize early due to the more unique skills needed for most of those events.
Some of this may also be influenced by cost. A lot of the more "minor" sports have some expensive costs involved (Rink/Pool time, Available Gymnastic Equipment + Huge Insurance).
Please don't take this as me supporting specialization. I don't. However, I do believe, at least based upon my experience in soccer, that players need a lot of touches on the ball at a young age to get comfortable with the ball. So a lesser athlete that gets more touches (when a multi-sport athlete is playing someithing else), is likely to be more comfortable and develop into a better soccer player. But ultimately, at the highest level, you need to have a superior athlete who has developed the comfort with the ball.
Taking that all into consideration, I think that the best soccer players are going to be those that spend a lot of time with the ball at a younger age. Soccer is likely their primary sport. But they also become (Post-Puberty) more athletic than their peers. In my world, I think that the specialization really can be held off until U-14/15 or so, and even then it shouldn't be done to the purposeful exclusion of other activities. I can't see any reason a kid couldn't participate in lower levels of other sports. But I do think it would be hard to play other HS sports.