Hard agree. I'd also argue that any meaningful antitrust action we may eventually impose on big tech companies should force this.
If the one thing so far that's led the feds to threaten this is that they wanted to build a modern protocol for cross-service messaging, then there's no sane reason we couldn't have asked for that exact thing as a spec.
I think what Google needs to do is to seperate the messaging protocol from the messaging software. The protocol needs network effects. The software doesn't. That's why shutting down Google Inbox didn't kill email and it's why any new experiments with email software can benefit from the network effects that email protocols already have.