As a Redditor pointed out, using a demonym that ends in '-ese' as a noun sounds incorrect or at least awkward (especially a singular noun--someone on the thread writes, 'For example you could say “I met a Chilean” but not “I met a Chinese.”').
Most of the countries with '-ese' demonyms seem to be in East Asia, and hence perhaps people in English-speaking countries didn't have much occasion to talk about people from them until relatively recently. (The exception might be the giant country of China, for which there was a separate nounal demonym that is now considered a slur.) That might explain why there is no natural-sounding way to refer to 'a Japanese', 'a Vietnamese', etc. But what about countries that have been known to English-speaking peoples for a very long time, e.g., Portugal, or the city of Vienna (or perhaps Malta)? Did earlier English speakers have another way of referring to an individual from Portugal or Vienna (or anywhere else I might have missed) other than 'a Portuguese' or 'a Viennese', and if so, what was it? (And, out of curiosity, was there ever a separate nounal demonym for, say, Japan, or Burma, in the Victorian period, like there was for China?)