Maybe the FAA will not make it "retroactive" but only applicable on manufacturing or purchase or registration after a specific date. I'm sure they understand that requiring virtually every drone over 250g to obtain RID seems like it would create a hardship for millions of people and wouldn't really be practical since you can be sure a huge majority wouldn't do it either from not being aware or not caring. Just like I'd bet there are millions of drones not even registered as required "by law." They would need quite a large "enforcement" team to knock on millions of doors then drag you off to a Federal Pen! They have more urgent things to be concerned about.