It's tricky: how is a user supposed to distinguish between "this site doesn't support HTTPS at all" and "a MitM is making the HTTPS site slow so I give up and use the HTTP version"?

What if it's their bank and they really need to pay a bill right now? Maybe the HTTPS site is just broken right now.

Users aren't wrong for clicking yes through these dialogs. This kind of UX is setting them up for failure with impossible choices. Its even worse in practice: users (including programmers, as evidenced by the comments in every HN thread about HTTPS) won't understand everything that's in play before making this decision. Assume users always click yes.

HSTS, preloading and restricting new features to HTTPS etc. are ultimately working to deprecate non-HTTP (inside browsers).