I think the current approach of allowing site owners to decide how they're going to support TLS is the right approach, browsers taking it upon themselves to enf

I think the current approach of allowing site owners to decide how they're going to support TLS is the right approach, browsers taking it upon themselves to enforce it is a bad idea. Some additional reasons why it is a bad idea have already been mentioned by other folks. When it comes to security, TLS is crucial to the security infrastructure of the internet as a whole; however most of the current security problems on the internet don't actually involve TLS vulnerabilities, even though those tend to make a lot of news in the technology press. The bar for exploiting TLS vulnerabilities generally requires setting up a legitimate seeming entity and injecting yourself into the internet traffic routing infrastructure, it is very much possibly (and indeed easy for state actors) but it is a much higher bar for common cyber criminals.