While Mexico is the only such country today other than the USA, it alone is good enough reason for people to avoid using "United Statesians" or the local equivalent thereof to refer to those of the USA. (Even worse is "norteamericanos". Why aren't Canadians and Bahamians constantly marching in the streets to protest against Spanish speakers' use of such a term to refer solely to denizens of the USA?)

The US, on the other hand, is the only country with the word "America" in its name. All the more reason for the use, in every major Western language other than Spanish, of America/American to refer to the US and its inhabitants.