Ehh FloJo and BJ were both specialized early in highschool. MJ only did track and no other sports, albeit not as seriously at a young age. Same with the East Germans.
FloJo, Ben Johnson, East Germans……….hmmmmm……not the best examples for the effectiveness of starting early intensive training. While I certainly agree that there would be a huge benefit to being able to very gradually build up an athlete's ability to handle training volumes and loads over an extended period of time; these athletes aren't the ones that should be championed for this benefit as their adult success was probably influenced by other factors.