As far as other sports impacting baseball, I would say certainly this has happened since the 1950's as many of the great athletes have gone to other sports. I just don't think this has had a lot of impact on the amount of teenage stars in the Majors as that was already in decline as the Major League game itself matured. I'm not sure if Michael Jordan would have become a star or not because he had such a huge strike zone and there is very little precedent for anyone 6'8" being a good hitter in the Majors although I believe Frank Howard was listed at 6'7".
Many people believe Bo Jackson was the greatest athlete to ever play in the Major Leagues and while they have a good argument, if you know baseball history, you know Jim Thorpe, voted the greatest athlete of the first half centuy in the 1900's, decathelon winner in the Olympics, pro football Hall of Famer, also played several years in the 1910's. In the context of his time, he was a greater athlete than Bo Jackson.