I've thought about this a lot, but there's really no way to get around the fact that you'll draw insane conclusions if you start from insane priors. And you're kinda forced into having insane priors if your parents believe insane things. Either we force people to make the right decisions (as decided by [insert favourite entity here]), or we live with some people doing batshit insane things. Those are the extremes of a spectrum that includes many only slightly forceful interventions, but in a nutshell, that's the choice we have.
That’s fine. It’s a free country. Their kids just don’t have a birthright to putting the public at risk. If they mostly stay at home, the damage is done; but it’s limited.
The other option, having the government override parents’ wishes around medical treatment, is rife with risk.