There are many OS interfaces that were deprecated after five years or even longer. It's been multiple times those five years since then and we'll likely have to deal with UTF-16 for much longer still. Having to provide backwards compatibility for UTF-16 interface doesn't mean they had to keep these as the defaults or provide new UTF-16 interfaces. In particular WIN32 already has 8-bit char interfaces that Microsoft could have easily added UTF-8 support to right then and re-blessed as the default. The decision not to do that was not a technical one but a political one.

This isn't "deprecate a few functions" -- it's basically an effort on par with migrating to Unicode in the first place.

I disagree you could just "easily" shove it into the "A" version of functions. Functions that accept UTF-8 could accept ASCII, but you can't just change the semantics of existing functions that emit text because it would blow up backwards compatibility. In a sense it is covariant but not contravariant.

And now, after you've gone through all of this effort: what was the actual payoff? And at what cost if maintaining compatibility with the other representations?