It's not just Windows and JavaScript. On Apple platforms, NSString is UTF-16. On Linux, Qt uses UTF-16 strings. Looking at languages, we have Java (which is where JS got this bug from) and C# both enshrining it in their respective language specs.

So it's far more pervasive than people think, and will likely be in the picture for decades to come.

ICU (International Components for Unicode, the library published by the Unicode folks) itself uses UTF-16 internally, and most of these things are built on ICU. I agree strongly with your conclusion--UTF-16 isn't going anywhere. I don't think the ICU people are even talking about changing the internals to UTF-8.

Qt could really change if they wanted to and really should have by now - it's not like they keep long-term backwards compatibility anyway unlike the others that you mentioned.

Of course they chose to integrate JavaScript so that's less likely now.