That was actually my first thought after reading this. Chances are, people in public wifi spots will actively and aggressively ignore any and all warnings that get in the way of them getting their work done (I have witnessed this far too many times).

However, Chrome surprised me the other day by not allowing me to continue on to a site that didn't have a valid certificate (I was MiTM myself using PAROS proxy to test something). In other words, we don't need stricter warnings, but the warnings must become errors for them to be noticed.