I either don't understand the scenario you are describing or I don't understand the failure mode you mention?
I normally don't pay much attention to whether a message is being sent out over iMessage or SMS, and less technical folks probably pay even less, so it's a bad situation when those two methods end up going to different people.
That said, I'm actually not sure what the best behavior for iOS would be here - I get that they want to use the "best" transport and send over iMessage rather than SMS if it's available. Ideally there would be some kind of warning if the phone number I have in my contacts doesn't match the one on the device that's going to receive the message, but that seems finicky as well (what if I only have their land-line?).
For awhile, at least at the Apple store, they were very deliberate about disabling "Find my iPhone" and iMessage when handing off or wiping a phone. I don't remember them doing this recently, so I figured it was built into the process now.
I do think we need some basic awareness about digital devices, just like people do when they let someone borrow the keys to their house (although, many people are terrible at managing that). I recently sold a house with a Nest and a few other IoT things. I wiped them and reset them to factory settings. The realtor and new owner were asking me for my login/password (I'm sure this happens often for them) because they figured it was still tied to my old account.
iMessage was launched in 2011 and archive.org shows that page first showing up in late 2014. I remember hearing very few options for removing your old phone early on.