There's no way we could ever stop the world and get everybody to use a new protocol overnight. Attempting to change the format of an existing protocol in-place

There's no way we could ever stop the world and get everybody to use a new protocol overnight. Attempting to change the format of an existing protocol in-place in a way that existing hardware and software cannot handle, would be an effort that's dead on arrival. The global transition to ipv6 is already hard enough, and that's with it designed as a separate protocol so that existing equipment that doesn't understand it can at least gracefully discard it instead of mishandling it.