> until Microsoft chooses to enable UTF-8 by default in all of their Windows editions.

I don’t know how likely this is. There are a lot of old applications that assume a particular code page, or assume 1 byte per character, that this would break. There are also more subtle variations of this, like applications assuming that converting from wide characters to ANSI can’t increase the number of bytes (and hence an existing buffer can be safely reused), which isn’t the case for UTF-8 (but for all, or almost all, existing code pages). It can open up new vulnerabilities.

It would probably cause much less breakage to remove the Best-Fit logic from the win32 xxxA APIs, and instead have all unmappable characters be replaced by a character without any common meta semantics, like “x”.