Not all sites exchange data worth the SSL-layer...let's say, 30-50% of the sites...that means for roughly half a user's non-email/social-networking web-browsing, they're going to either be saturated with a warning message that, if psychology research is to believed, is going to be promptly ignored like your apartment's super-sensitive smoke detector or the boy watching out for wolves.
What's the actual reasoning in OP's mind here? That the average web user is someone who, when the computer gives some warning sign that is disconnected from any kind of discernable threat, that the user will automatically take it seriously? That they'll demand to know why they're in danger, and then will rise up and petition the Internet regulatory commission to make all sites do what it takes to make that red icon go away, and the web will be more secure?
I don't want to be overtly negative towards well-intended idealism, but this is such a bizarrely naive viewpoint that it must be openly challenged. That, and it just blithely ignores all the research showing what happens when humans are overloaded with impotent warning signs. This is the kind of idealism-without-consequences that can end up causing more harm than good.