Overall, Hyson feared that we were creating a vicious cycle. Kindergarten might continue getting more advanced, causing the average age of kindergarten students to go up in response, which in turn would allow the academic demands to be ratcheted up further. Eventually, after much turmoil, kindergarten might turn out to be simply first grade under a different name, with the same curriculum and the same age students as first grade traditionally had.
For example, watching Jimmy Clausen be the top high school football recruit in the Class of 2006 because he was a 19-year-old quarterback chewing up 17-year-old defenses was eye-opening. If he hadn't been red-shirted twice by his NCAA football-savvy parents, he would have been a young 17-year-old in the Class of 2004. Would he have gotten a scholarship to a high profile offensive college then?
Of course, your mileage may vary.
Unfortunately, we don't seem to know how true this theory behind redshirting is. It's hard for social scientists to find out anything very definitive about child-rearing practices because parents are constantly adjusting their decisions according to what they see as the specific (and often changing) needs of their children, which is good for their kids but bad for social science studies. Jim Manzi would recommend running a randomized experiment with a control group, but outside of welfare moms, it's hard to find parents who will sit back and passively let social scientists treat their children randomly.