Enforcing what? It's up to your email service to adopt the change. The way it currently works was probably okay for the early days of the internet when people had just started using emails, but now that so much private and sensitive financial information passes through your email account, it makes little sense that the same credential you use to login to your email account (email id) is also what you must use when some random website forces you to register in order to access it, just so they can send you massive volumes of unsolicited spam.

Certainly a simpler change with more potential to do good relative to the ease of enforcing a convoluted passkey-slash-usb-slash-save-this-file-somewhere-safe implementation