Using just "United States" as country designator seems very unwise to me. I think that it could lead to confusion. Since 1891 there's also been a United States of Brazil. Look it up on Wikipedia. And then there's the United Mexican States. Not an exact match, but possibly confusing.
And if the EU ever renames itself to the United States of Europe, that hypothetical change could further confuse things. See https://en.wikipedia.org/wiki/United_States_of_Europe
for a discussion of that possibility.
After all, WikiTree is a world-wide tree. Are there other countries called the united states of something? We should be more precise.
Here's my 2 cents' worth. I think WikiTree, FamilySearch, etc., should standardize on either United States of America or USA. USA is my preference, and that's what I try to use for all my profiles. It's also nice and short.