> In general, "just use UTF-8" is a shorthand for "just let me pretend non-ASCII doesn't exist", because it leads to people writing code that assumes one byte == one character.

Only if people writing code are entirely oblivious to what UTF-8 is. The most basic feature of UTF-8 is that it's a variable-length encoding scheme. If the main feature is it's variable-length, why would any programmer assume the length doesn't vary?