Usernames and passwords are always 7-bit ASCII at least in Japanosphere, to the point it would look odd that you get to log into a computer using their own legal names. To use a computer for any useful purpose as a Japanese, you will have to be able to understand what "English" or "alphanumeric" or "half-width" means, which are non-tech terms for `char str[]`, and be able to constantly and quickly switch IME between ASCII mode and multibyte mode with an at most 2-key combination.

It might be a "you're holding it wrong" situation, but everyone has already learned how to hold it "correctly " - it'll be a disruptive change to default a "natural" hold that you suggest.