I always have to explain that we didn't come up with the name, the British did when we were first a colony, almost 500 years ago. Then the US became the first independent nation in the Americas, so it naturally became the nomenclature for us as a people. If the latin American countries wanted to have claim on the name, they should have had a revolution first instead of copying us decades later. It's not our fault they decided to live under European tyranny for decades longer. If their ancestors weren't so passive, then they would be called Americans, not us. You snooze, you lose.
I say all this just to be annoying, of course. When I'm talking to a Latino in person who complains about it, I explain it all in fluent Spanish to make sure my point is well taken.
Every country in the world has a name for its people, and that just happens to be ours and it's not going to change any time soon. Besides, not even Spanish speakers enjoy using the word "estadounidenses".
My wife is Mexican. Us calling ourselves Americans drives her crazy (so I make a point, every time we visit her family in Mexico, of loudly declaring, "well, goodbye America!")
Latin Americans feeling perturbed by the 'American' demonym is certainly understandable.
Nevertheless, I feel that 'USian', despite being motivated by an understandable disgruntlement, is used/proposed in bad faith for these reasons: 1) 'American' is the demonym a consensus majority of Americans have have chosen for themselves. 2) 'USian' only works in textual conversation, in spoken conversation it is one small mumble or tongue fumble away from 'US Asian' aka Asian-American. 3) Combining an uppercase initialism with a lowercase suffix is unnatural, not idiomatic English. The clumsiness of this term seems deliberate. 4) It is rarely used except in the context of bitter remarks about Americans (such as above.)
These four factors combined lead me to conclude that the term exists and is primarily used for trolling Americans.
We have a word for this in English: Americans.