It's easier on Android where anyone can "become" the SMS app and integrate all your chats in one place. Apple does not allow that, so you're always going to end up going back to iMessage with some people if you just know their phone number and try to send an SMS.
Sure, but "the old one is no longer available" is absolutely going to get everyone to switch in the approximately 60 seconds it takes to go from the notification to having a new account.
I mean, Google basically does this every time they change the default Android messaging app (though in fairness they do get a huge advantage of already knowing all your contacts): https://arstechnica.com/gadgets/2021/08/a-decade-and-a-half-...