Every time a parent who cares about their child’s education pulls them out of public school, that’s one less parent who might otherwise be involved enough to improve the public school situation. You end up with public schools containing only the children of parents who are unable to afford the time or money for other options, only the teachers who can’t get jobs at private schools/tutors, etc etc.
It becomes a self-fulfilling prophecy for everyone to think “public schools are bad, I’m going to send my kid somewhere else.” I really wish we’d focus that energy towards making public schools better.
* It would probably take an emperor to do this, because there’s no way such a law would pass in the US today.