That's completely spot on. Right now, if your email provider doesn't have native support, we ask you to create a persona.org account so that Mozilla can vouch for you. In the future, this goes away.
Similarly, the UI is all displayed in response to navigator.id.* functions. If a browser implements those natively, the Persona UI at login.persona.org completely goes away.
The more successful Persona is, the less Mozilla is involved in the login process. :)
Then you'll start seeing an authentication page hosted by Google, instead of the Persona fallback. Native options are always tried first, both for client-side navigator.id functions, and for server-side authentication. If one of those is missing, then login.persona.org fills in for that component.
Similarly, the UI is all displayed in response to navigator.id.* functions. If a browser implements those natively, the Persona UI at login.persona.org completely goes away.
The more successful Persona is, the less Mozilla is involved in the login process. :)