Specialization is a fact for kids who want to succeed in sports, except that it leads to more injuries and more stress for parent and child, sometimes to the point where kids leave athletics behind them before puberty.
It is made nearly necessary by the programmatic progression from pee wee leagues through town, travel, regional and junior Olympic teams. The competition for high school spots and college scholarships is doing it. Do you help your kid become normal or encourage them to become super? (In individual sports — e.g, tennis, gymnastics, swimming — the choice has to be made by the age of 7 or so, if the highest level will be achieved.)
Get left behind at any level and the child doesn't get the best coaching, the biggest profile, the top competition. All of which help continue progress. In short, if your kid misses the train at the first or second station, it will get too far ahead and s/he'll never catch up. It's not that sports won't be fun later on. It's just that — with very few exceptions — the top-level college (i.e, scholarship) or pro experience will be unknown. Ironically enough, pro athletes have found that once they have achieved the highest levels, it is cross training that lengthens their careers.
As if that is not enough you can also worry about redshirting beginning in kindergarten to encourage/discourage academic as well as athletic success.
** How long do you wait to turn your child into a robot and you into a dictatorial taskmaster? And if you don't have you failed or served your child? **