HTTPS proxies work less well today than they did even five years ago. Certificate pinning and other security improvements have broken a lot of things (by design).

So the question is: What is more important, allowing HTTPS proxies, or stopping governments with a CA from MitM-ing traffic (e.g. Iran, China, etc).

A nice compromise might be to inform users that they're being MitM-ed by an installed CA, but only once and subtly so.