This is only half right. "You're a gentleman" is still a common, and I suggest perfectly serious, way of expressing thanks. The people who readily shy away from the word are the remains of the old leisured classes who dislike being thought snobbish. Harold Laski satirised the breed in 1932 by saying that the gentleman, though courageous, "maintains towards life an attitude of indifferent receptivity"; it was "vital that he should belong to a club and desirable that his views should coincide with those of the Morning Post". He "profoundly respects the Royal Family (of whose failings he breathes no words in public)": and "he may become a director of a company, provided he is not too well informed about its business".
Laski wasn't the first to question the connection between honour, modesty etc, and mere rank, and to mock the idea that the gift of commanding others is conferred by the possession of independent means. James I, asked to give a baronetcy to an ambitious subject, replied that he could make a man a baronet but the devil himself couldn't make him a gentlemen.
The Latin gentilis meant "of the same people" and the notion of ranks must have come first, yet the answer to "what is a gentleman?" has always been vague. R L Stevenson said anyone could see that Shelley was a gentleman and Byron was a cad: but there wasn't much sense in it.