The majority of the big sites that people use (Google, GMail, Youtube, Facebook, Reddit, NYT, WaPo, etc.) are already being served using HTTPS. I think if a couple of HTTP sites an average user still browses start showing these warnings they will notice them. And what matters, the owners of those websites will notice them and will ask their "IT guy" hey "why our website is marked as insecure? I want a green lock like Gmail has".