> The most correct way to expose Unicode to a programmer in a high-level language is to make grapheme clusters the fundamental unit, as they correspond to what people think of as "characters".

> UTF-8 is a non-starter because it encourages people to go back to pretending "byte == character" and writing code that will fall apart the instant someone uses any code point > 007F.

These are somewhat different concerns, you can provide cluster-based manipulation as the default and still advertise that the underlying encoding is UTF-8 and guarantees 0-cost encoding (and only validation-cost decoding) between proper strings and bytes (and thus "free" bytewise iteration, even if that's as a specific independent view).

> Far too many real-world uses for textual data require the ability to do things like length checks, indexing and so on

This is not a trivial concern e.g. "real-world uses for length checks" might be a check on the encoded length, the codepoint length or the grapheme cluster length. Having a "proper" string type doesn't exactly free you from this issue, and far too many languages fail at at least one and possibly all of these use cases, just for length queries.