Meanwhile if you spent 15 years in Canada and got Canadian citizenship, no one would care if you started calling yourself Canadian, unless you're a really bad culture fit.
I think it would be similar in Australia, where I'm from, but different in any country where "nationality" and "linguistic/cultural/ethnic background" are synonymous for most people. Most of us wouldn't call an American dude living in Thailand for 20 years "a Thai person" either. The "you can't become Japanese" thing often gets held up as an example of unique or unusually strong Japanese xenophobia, and I don't think it's particularly unusual, though we (humans in general) could probably afford to get more precise about our thinking with regard to nationality vs ethnicity.
I think it would be similar in Australia, where I'm from, but different in any country where "nationality" and "linguistic/cultural/ethnic background" are synonymous for most people. Most of us wouldn't call an American dude living in Thailand for 20 years "a Thai person" either. The "you can't become Japanese" thing often gets held up as an example of unique or unusually strong Japanese xenophobia, and I don't think it's particularly unusual, though we (humans in general) could probably afford to get more precise about our thinking with regard to nationality vs ethnicity.