> It just refuses to work with non-UTF-8 files at all -- is this less sloppy?

You can debate whether it is sloppy but I think an error is much better than silently corrupting data.

> The best approach is to treat data as opaque bytes unless there is a good reason not to

This doesn't seem like a good approach when dealing with strings which are not just blobs of bytes. They have an encoding and generally you want ways to, for instance, convert a string to upper/lowercase.