Apple could easily open the protocol, but they won't. They also won't use any of the other ones so as long as you'll be using iOS, you'll be in the iMessage garden and everyone else will be in another garden.
I'm hoping that's what RCS[1] will be (ids? And that's why I referenced it above). It's supposed to by the successor to SMS, but it's been around for quite a while and not adopted, so I'm not sure what's going on with it.
Edit: It looks like as of a couple months ago it's live in the UK and France, and just those two countries.[2]
1: https://en.wikipedia.org/wiki/Rich_Communication_Services
2: https://9to5google.com/2019/07/01/rcs-hands-on-not-quite-ime...
Whatsapp really seems to be missing the boat with tweens and teens - that’s cant be good for their future.
With my "green bubble" friends, we usually use Telegram or Signal or Keybase instead.
I'd love to see a standardized IM/Text protocol instead of 1000 fragmented platforms. I've got on the order of 10 chat apps on my phone right now, all used in different contexts.