"So it will be presented with the 'this page (attempts) to be secure' UI, including whatever scary warnings are needed if things seem broken."
. . . but now I don't follow you. Say you try to connect to your bank. I intercept the connection during the TLS handshake. The request never reaches the bank, so you never get the bank's certificate. You get my self-signed certificate instead, which appears to come from the bank's website. In this case you clearly want a warning of some type, or else you have no protection against MITMs at all. But how does the browser distinguish between this, and the case where the site's legitimate owners are using a self-signed cert?