If we accept that the world has moved to webmail, and use a GUI client, then the way to make it easier is bake in into the client and make it seamless so there's no manual futzing with anything. Make it like TLS certs, so there's a padlock icon for encrypted mail, yellow for insecure, and mail that fails validation gets a big red warning.
Unfortunately, purists in the community could not accept that, so it's never happened, and so gpg failed to get critical mass before alternatives popped up.