SMS is sort of that, right, at least a crude form?

Yes, why shouldn't the next standard (like 6G) introduce support for a better messaging network than SMS? With group chats and image support. Each carrier could decide if you can send unlimited messages or a few thousand. In any case it would remove this dependency on a single company.

Edit: Just realized there's RCS: https://en.wikipedia.org/wiki/Rich_Communication_Services

Such a standard is unlikely to ever introduce end-to-end encryption.

It already exists and is enabled by default (at least on my Pixel phone using the default messaging client).

https://support.google.com/messages/answer/10262381

That's not part of RCS, that's a Google extension.

iMessage is the working implementation, as long as nobody joins with green bubbles [0].

[0] https://www.androidauthority.com/green-bubble-phenomenon-102...

yes, but I don't think SMS is sophisticated enough. After all, instant messengers replaced it ...

...or use Tox and no service providers would be affected because it's 100% decentralized.