They did realize it. There are two mechanisms for handling this issue - turning off iMessage in Settings before deactivating your iPhone, and calling Apple after deactivating your iPhone. Both are documented.
There is no way for your iPhone to know it's been deactivated and still communicate with Apple. If anything it's the fault of the carriers for not providing Apple with an API hook for "this iPhone was just deactivated."
Also SMS is many times more lossy than iMessage. I've had messages show up to my friends on AT&T several days after they were sent, and not all. SMS messages longer than the length limit almost always arrive out of order, and sometimes parts don't arrive at all. Relying on non-iMessage texting for medical assistance is stupid to begin with.