This is what I believe Google is actually trying to get carriers to do, and I suspect carriers (in some shape or form) will actually do this, just not in the way you think.
RCS will eventually become the dominant messaging standard, however, I think they’re actually working on a backwards compatibility for SMS and MMS in some capacity. In this way, phones (like the iPhone or older Android phones) will still be capable of sending and receiving SMS and MMS in typical elitist walled-garden fashion, but the carrier will receive it as an RCS message and relay it to an RCS-compatible device as an RCS message.
In this way, group chats with four Android users and two iPhone users will still allow those Android users to benefit from RCS from each other (typing indicators, reactions, potentially some level of E2E, support for large media, etc), while the iPhones in the group chat will actually be the ones having a negative experience (no typing indicators, reactions appearing as text messages, no E2E, obnoxious green bubbles) since Apple refuses to integrate RCS into their Messaging application. Of course Apple will continue to gaslight their customers through high contrast green bubble dark patterns, and continued refusal of adopting RCS or creating iMessage for Android. As they’ve made clear, they don’t care about giving their customers the best possible experience, and prefer to maintain market control for as long as possible.
The #GetTheMessage ads are likely gearing up for the eventuality of this change, and the Pixel x iPhone ads are all “buddy buddy, kill them with kindness” so they can out Apple as the hostile ones when they refuse to acknowledge the existence of other smartphones either through its aggressive marketing, or through refusal to adopt open standards.
If this were all to happen, depending on how well the RCS backwards compatibility worked and its ability to out Apple as the shut ins that they are, I could (crazy talk) foresee Apple creating a standalone iMessage app to, at the very minimum, keep Android users talking within their iMessage ecosystem.