What you've re-invented is XMPP.

The original Facebook Messenger and Google Talk both used XMPP, it has support for encryption and push notifications.... For a brief period, you could actually chat across ecosystems.

And it died, everyone closed up their ecosystem.

We do have matrix now, but it's still largely irrelevant, and doesn't really feel fully baked yet.

At this point, all the major companies have a huge vested interest in keeping things closed.

Without blue bubble lock-in, I, and quite a few people I know, would ditch increasingly mediocre iPhones for Android, so apple has to keep building iMessage exclusive features and has to avoid ever releasing an iMessage android app (most recently, Apple Invites, which integrates with iMessage cleanly and is impossible for third-party apps to integrate so neatly).

I expect Apple to continue to leverage "Apple Intelligence" as a feature that only integrates well with iMessage so that they can continue to lock users in, and keep the conversation as far away from open chat protocols as they can.

In the AI age, unencrypted textual conversations are a new source of training data, so Instagram, Twitter, and Google want to keep their own messaging systems to themselves.