It is like any system there are always a few who have less the pure motives most are just doing the best they can (think social welfare).
It seems to me if sports were organised by age not grade parents would not err on the side of caution so much. I do think still being at school at 19 is not a good thing.
We live in Europe and all sports are organized by age, not grade. While kids still have to share PE class with the kids in their grade regardless of age, wherever there are standardized norms (swimming, athletics) kids are graded against age norms - so, being accelerated, I'd learn shot putting with everyone else in my class, then was the only one to still have to throw a ball for my grade, and for sports day I was always grouped with the kids from the grade below due to my year of birth.
However, there is still red shirting, and it's getting worse. Recently, there was a ridiculous class at our local elementary where a whopping 11 out of 20 kids were redshirted, boys and girls, and none of them had actual delays, it was all for the "gift of another year" (a year in which, oddly, most of them had learned to read regardless, and were way ahead to the kids who started on time). A friends daughter with a June birthday, more than a year younger than most, struggled badly. Finally, she had to hold her daughter back a year in fifth grade, too.
It skews the system, badly. I wish they'd ask for evaluations of kids who are held back and unless a child has an actual delay which can be expected to be remediate by the sheer passing of time rather than special ed services (preemies come to mind) either have them enter on time or put them in the higher grade where they ought to be.