I’m not talking about your father but I really doubt that the millions of people running 6+yo phones would care a lot about this. They could still send SMS, it’s not like they would became suddenly unreachable.
Apple could even be generous and still allow group chats or whitelist existing accounts.
Also, you have to take into consideration that iMessage is pretty US centric, the rest of the world wouldn’t really care about this.
The whole point is moot, even though I agree with your take.
There were plenty of confirmation from other users in this thread having their out of the most recent iOS version iPhones (someone mentioned their iPhone 5 currently running iOS12, with the most recent one being iOS17) working just fine with iMessage.
Apple could even be generous and still allow group chats or whitelist existing accounts.
Also, you have to take into consideration that iMessage is pretty US centric, the rest of the world wouldn’t really care about this.