Aren't those keyboards usually readily switchable with a standard US keyboard layout (e.g., in Windows, MacOS, or Linux)?
Do you have any direct experience with this?
My point is that 7-bit ASCII underlies virtually all other keyboard / character encodings. Among the few cases where it doesn't (e.g., IBM mainframe EBCDIC, independent of ASCII), the principle encoding remains the Latin alphabet, and can readily be converted using standard system utilities.
They aren't readily switchable unless explicitly configured to be, and yes, I am speaking from direct experience.
I get your point, I am just saying that it is not universally true, even if my example is a stretched one (Cyrillic keyboard users always configure Latin-based layouts too, but it's not a default, and it's easy to forget esp if you install from a fully live env configured with both).
If it's still not clear, you can end up on a login screen with an ASCII password you can't type in, especially so without the GUI.
In what platforms is it not possible to input various Unicode or emoji characters, whether at all or reliably?