You can make a seamless experience so that people don't even realize that their email is encrypted, but then the problem becomes: if their computer crashes and

You can make a seamless experience so that people don't even realize that their email is encrypted, but then the problem becomes: if their computer crashes and they lose their private key, they have now lost access to every previous email ever written. That's just unacceptable. I closely guard several backups of my private key, but most people aren't going to create an enormous single point of failure like that.