Comment by montagg

Basically what iOS messages does with SMS. I would note that this hasn't resulted in everyone using iOS messages as a communication protocol, but it does make users aware when it's not in use, with some inherent social pressure.

When dealing with any kind of network effect and existing user behavior, though, especially one as ingrained as email, I would highly recommend being judicious about using the word "just," given that these are mutually-reinforcing effects. Imagining a solution is a world away from implementing one.

Still, I think you're on the right track: create a new protocol on top of an old one, use the same input methods and user behaviors, call attention to when someone is not using it (e.g., green bubbles), and make it extremely easy for existing services to migrate to use it (i.e., the standard/standards body, notably NOT what Apple has done). One part that's missing here is some pressure to make this happen: positive pressure (economic, product sense, persuasion) or negative (economic, social, regulatory).