I'm not so sure about the "offensive" angle - it could be the other way, that is, first Japanese as a noun stops being used (for some reason), and then because

I'm not so sure about the "offensive" angle - it could be the other way, that is, first Japanese as a noun stops being used (for some reason), and then because of that, using Japanese as a noun starts to sound weird, and then offensive. Americans have had plenty of racist/hostile opinions against Mexicans or Russians, yet "a Mexican" or "a Russian" remains totally fine.