If you allow the most wealthy and powerful segment of society to opt out of public services, those public services will get worse. The reason is simple: You have removed the incentive for the most influential people to use their influence to improve public services. Indeed, we see this dynamic in action everywhere we look. If rich people can all go to a private hospital, the public hospital is worse. If rich people can buy their way out of the TSA security lines, the experience for the regular people who have to wait gets worse. And when the rich people’s kids all go to private schools, the public schools are worse than they would have been if they educated everyone’s kids.

From the perspective of an individual parent, the purpose of school is to give their own kid the best possible education. But from the perspective of society—from the perspective of an architect of public policy—the purpose of schools is to give everyone a good education. Wealthier parents often observe that poorer public schools are full of poorer students with more severe needs. They respond to this by opting out and putting their kid into a private school where the student population has less severe needs, and more resources to address them. This may be rational if the goal is to give their kid the very best, but by isolating the neediest children in poorer public schools, it produces a worse outcome from the perspective of public policy, where every student is equally important.