Flutter's edge over these technologies is faster development cycles (very fast build times, sub-second reloading without losing state, very rapid ability to update code/GUI after deployment to a testing device, and very fast render times).

Xamarin and Qt are fine products, and I know Xamarin has been working on some improvements over deploy times, but it still can't touch what Flutter does there. And if you know C++ and C# already, Dart is a breeze.

A programming language is more than just semantics, Dart eco-system is a tiny dot when compared against C++ and C#.

Flutter deployment times aren't worthwhile enough to bother with a language with uncertain future.

Doesn't Dart interop with JavaScript libraries?

It does, given its history.

However that is very little against C# and C++, which can also be compiled to JavaScript/WebAssembly.

Additionally there are TypeScript, ReasonML, and a myriad of other languages with JavaScript interop.