My son played for a coach that never took a full team of graduating seniors for two reasons, the one stated above and that it was based on helping rising seniors and graduating seniors obtain scholarships through exposure and not about winning. He also made it quite clear about his expectations, if you had other things to do, either do them or not play, and he did carry more than he should after he knew that some wouldn't make it through, or had some obligations that he was told ahead of time. With a 68+ game schedule, he needed to know he had enough players to cover. While you are blaming kids for not full filling their part, seems to me the coach didn't do his job in planning ahead, or making it clear what was expected to players AND parents. The blame goes to each side, players, parents and the coach.
He also had "invited" tryouts, even knowing who he was going to take. I don't think my son ever did NOT try out for any team, it sort of goes along with you have to earn a spot. I know that many coaches take players upon recommendations, but my way of thinking is that when you just "give out" something, you aren't actually teaching them what they need to learn early, that in this game you "earn" it. And this game is also about commitment as well.
Why should one remain with a team if he feels he didn't have to work hard to get it in the first place? JMO.