The only valid reason for still having files stored in legacy ANSI encodings is that their only use is input to software that has not been maintained for ~30 ye

The only valid reason for still having files stored in legacy ANSI encodings is that their only use is input to software that has not been maintained for ~30 years and cannot be updated. That's fine because they're just binary inputs in a closed ecosystem that no one touches. But if they are supposed to be treated as text, then yes it's the text that's buggy - they should just be converted to UTF-8 once and have the originals thrown away. UTF-16 is something that Microsoft has cursed us with by inserting it into specifications (like USB) so that we cannot get rid of it, even if it never made any sense what so ever. But those are in effect explicit protocols with a hard contract, very different from something where you would "assume an encoding".