If it's turtles all the way down and at every level you use utf-8, it's hard to see how any input with a different encoding (for the same underlying text) will

If it's turtles all the way down and at every level you use utf-8, it's hard to see how any input with a different encoding (for the same underlying text) will not be detected before any unintended side effects were invoked. At this point, I don't see any sufficiently good reason to not use utf-8 exclusively in any new system. Conversions to and from other encodings would only be done at well defined boundaries when I'm calling into dependencies that require non utf-8 input for whatever reason.