I'm a proponent of creating separate public/private playoff divisions not because I believe that private schools win CIF too often or some such, but because I believe that public and private schools play by different rules when it comes to students attending school. Privates, to the best of my knowledge, do not have an attendance area and must draw students in. Public schools, almost without exception, have an area that feeds into the school.
Given that, I wonder if many of the ills, railed against by people like myself, with how high school sports have changed over the past decade or so actually stem from the removal of the larger private schools from a mixed league and with the creation of the Trinity League. Up until the recent changes to the playoff format/division placement, the Trinity got 3 teams into the playoffs and likely a 4th. This put tremendous pressure on coaches in the Trinity to win or risk getting fired at a school where the desire to succeed in sports was seen as a way to attract kids and fill seats (which isn't to say that public school coaches don't get fired for not winning....). So you recruit. Run youth camps. Build bigger and more expensive facilities. Partner with groups to get kids on your campus who can play and help you win. Hire coaches whose sole job is to go out to watch youth teams play. Pay coaches big money to make sure you win, and fire them when you don't.
Had these schools remained in mixed leagues would that pressure to win at all costs have existed? It certainly seems to have created an arms race between schools in that league which has also had an outward push to other schools trying to keep up.