UTF-8 everywhere is not great and UTF-8 in practice is hardly ASCII-compatible. UTF-8 in source codes and file paths outside pure ASCII range breaks a lot of things especially on non-English systems due to legacy dependencies, ironically.

Sure, it's backward compatible, as in ASCII handling codes work on systems with UTF-8 locales, but how important is that?

> as in ASCII handling codes work on systems with UTF-8 locales, but how important is that?

It's only Windows which is stuck in the past here, and Microsoft had 3 decades to fix that problem and migrate away from codegpages to locale-asgnostic UTF-8 (UTF-8 was invented in 1992).