Earlier quoted context omitted.
Why, exactly, would Apple and Google not be able to fallback to SMS? Without involving carriers at all? It's not like there's any real benefit to having the carriers in that conversation, you can already text from either device just fine. That would continue into the future.
SMS is not transferred over the internet. It is transferred over a telecoms network. Only the carriers can deliver a SMS message. I guess it is possible to deliver a message over the internet to a SMS gateway somewhere, but as SMS is telecoms technology, only internet is not enough.
Additionally, 5G SMS does generally go over IP as SMSoIP: https://www.iplook.com/info/what-are-the-changes-of-sms-in-5...