In the end, the only layer at which it really matters whether your byte sequence can be decoded is the font renderer, and just being valid utf-8 isn't good enough for it either.

Having "accents fall off" has gotten people murdered [0]. Accents aren't things peppered in for effect, they turn letters into different letters, spelling different words. Analogously, imagine that a bunch of software accidentally turned every "d" into a "c" because some committee halfway around the world decided "d" should be composed of the "c" and "|" glyphs. That's the kind of text corruption that regularly happens in other languages when dealing with text at the code point layer.

[0] https://languagelog.ldc.upenn.edu/nll/?p=73 . Note that this is Turkish, which has the "dotted i" problem, meaning that this was more than likely a .toupper() gone wrong rather than a truncation issue.