In practice, 99% of use cases do not have to worry about a MitM attack that is successful 100% of the time. The probability of a MitM attack remaining successfu

In practice, 99% of use cases do not have to worry about a MitM attack that is successful 100% of the time. The probability of a MitM attack remaining successful for all your interactions (and that's what would be required for it to go undetected) drops exponentially under a lot of (though admittedly not all) attack scenarios. This is exactly the reasoning behind proposals such as TACK. Edited to add: Keep in mind that external verification basically never happens, as much as cryptographers want it to happen. In the real world, you have a choice between no verification at all and a pinning-based verification which is weaker than the theoretical ideal, but makes MitM'ing significantly harder and much more likely to be detected for everybody at no user interface cost.