[imagist]

I don't think you need to even concede the idea that users are rational actors--there are plenty of reasons why a rational actor would prioritize another factor over security. For example, many people got Yahoo email addresses a long time ago, and built a personal contact list of people who only know their Yahoo email. A rational actor might value keeping in contact with those people over their privacy. That doesn't mean that it's okay to expose that person's data.