One requires one entity, the US government, to make a decision. The other requires millions of entities to make a decision. For this case, the government making the decision makes more sense IMO. Every restaurant, coffee-shop, supermarket etc, doesn't have to do anything (they're already on DST). Everything is already happening. Deliveries are already scheduled for opening hours etc... Your suggested way would require millions of not billions of little coordinations.

That said, every old non-updated OS is going to F up once this happens.