There are definitely a few exceptions, but by and large it is difficult to be multi-sport the further you progress.Bo Jackson, Turner Gill, Deion Sanders and many others beg to differ.
If it takes 10,000 hours to master a certain skill, there is only so much time available for a person to work on those skills. Guys like Bo and Deion had athleticism that was generational, which helped them out a lot. There is definitely a point at which it makes sense to focus on one sport, I just don't believe that time should be middle school, or even really high school.