Am I missing something, or is this suggestion trivially mitm-able? If you fail to forward the request the upgrade, neither side will know something is missing.
I think you're missing the part where a non-upgraded connection will be treated by the client as insecure and untrustable.
So the suggestion is that in 1994 (when the https protocol was introduced) we should have put big scary warnings on every non-ssl site? I don't see how that could've gone well. It's now 22 years later and people still aren't ready for that.