I think you are mistaken. By the original definition the US could never be anything but a first world country. In modern usage first-world and third-world are a

I think you are mistaken. By the original definition the US could never be anything but a first world country. In modern usage first-world and third-world are anachronistic ways of saying developed or developing economies. I don’t see any possible argument for the US not having a developed economy as long as the dollar is the world’s benchmark currency.