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.
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.