Thats obviously a no-starter. Its not expensive because its "snuck into the control channel", which is only partially true. If that was the case and it would be immensely expensive to transport data that way, there would be a technical solution in no time.
Remember that before SMS, most payload was sounds, voice communication. Even with codecs that is still much more bandwidth intensive. And don't even get started with routing voice cross-carrier with minimal latency; atleast with SMS you can still temporarily hold them while you wait for capacity to free up.
What's funny is that the market has tried to solve it. The first cell phone I ever got offered low-cost AIM. My work BlackBerry has a free messenger app. But it turns out people like sending messages to a phone number, any phone number, and don't care for using multiple ways to text. So SMS is the only universal standard for this. Modern phones could re-route the data to save money but the receiving phone might still use the old protocol. It's a literal "network effect".
>Modern phones could re-route the data to save money but the receiving phone might still use the old protocol.
The iPhone messages app does exactly that. It first tries to send the message via Apple, with fallback to SMS if either the remote phone isn't an iPhone or if it can't establish a connection to it immediately.
Abstraction is the right answer. Just send a message, and let the application layer figure out the right transport to make it readable on the final device.
That may be the Messages.app (on iOS and even more on Mac) plan. I sure wish iMessage and BBM were made interoperable via some well-defined bridge standard.
Remember that before SMS, most payload was sounds, voice communication. Even with codecs that is still much more bandwidth intensive. And don't even get started with routing voice cross-carrier with minimal latency; atleast with SMS you can still temporarily hold them while you wait for capacity to free up.