What are you talking about? It’s possible to send group SMS messages on iPhone.
If everyone has it turned off, it is just the normal group SMS experience (also terrible). SMS group chat isn't gone, you just very rarely see it because there is almost no reason for an Apple user to disable iMessage.
This is also no different to everyone in a group who can use WhatsApp or a similar app using that.
SMS is the lowest common denominator fallback. RCS is not much better.