Which is why Python uses a solution that ensures fixed-width data. There's never a need to worry if a code point will extend over multiple code units of the internal storage model, because the way Python now handles strings ensures that won't happen.

I really think a lot of your problem with this is not actually with the string type, but with the existence of a string type. You want to talk in terms of bytes and indexes into arrays of bytes and iterating over bytes. But that's fundamentally not what a well-implemented string type should ever be, and Python has a bytes type for you (rather than a kinda-string-ish-sometimes type that's actually bytes and will blow up if you try to work with it as a string) if you really want to go there.