> And most mainstream GUI toolkits are, as well.

No. Windows use UTF-16 internally. Most GUI toolkits do not.

> It can be said that UTF-16 is the de-facto standard in-memory representation of unicode strings, even though some runtimes (Rust) prefer UTF-8.

No, that wouldn't be true at all.

Your technical merit seem to be limited by your Windows experience, and even that is dated.

Microsoft recommends UTF-8 over UTF-16 since 2019 [1].

1: https://learn.microsoft.com/en-us/windows/apps/design/global...