[Avernar]

Semantically equivalent yes, access time equivalent for variable width strings no. One of the reasons for Python 3's odd internal string format is because they wanted to keep indexing and have indexing be O(1). The reason why I think replacing indexing with iterators is that it removes this restriction and they could have made the internal format UTF-8 and/or easily added support for graphemes.

I prefer to have a system where 100% of the cases are valid and teaching people corner cases is not required. We all know how well teaching people about surrogate pairs went. And we're not forbidding the 99.99% case but providing an alternative way to accomplish the exact same thing. The vast majority of code uses index variables as a form of iterator anyways so it's not that big of a change.

The main reason people keep clinging to indexing strings is that's all they know. Most high level languages don't provide another way of doing it. People who program in C quickly switch from indexing to pointers into strings. Give a C programmer an iterator into strings and they'll easily handle it.