In addition to UTF-8, my language happens to have ~2 additional code pages/Latin based encodings. Some websites still serve (or very recently used to serve) text files in such broken encodings, so I have to convert such files before use. It's deeply unpleasant. Windows has supported UTF-8 in some fashion for over 15 years, get with the program people.
(I would make an exception for preserving historical non-UTF-8 files in their original byte-exact form, for the same reason that I wouldn't digitise an analogue photograph and then burn the original - but let's be real, all such files have been created by now)
File longevity wins over grammatical correctness most of the time for me. I have text files going back to the 80s, so I'm glad I didn't use any fancier software to write them as they'd be completely unreadable today.
I suspect for nearly every other language (or at least any language that doesn't use the ~100 characters/symbols used in the English alphabet), old ASCII text isn't terribly useful.
But maybe we should all use monochrome bitmap files for everything? That would be very simple.