alt Hacker NewsDart is the language flutter (the gui toolkit) is written in.
I've no idea what you mean regarding: "rather than exemplifying its flexibility and adaptability to _your_ platform, programming language etc".
It can produced mobile, web, or desktop apps from the same codebase... Dart and flutter are open source projects supported by Google, like android and go lang.
Seems like you're looking for excuses not to use dart/flutter. That's fine but you could just say you don't want to use them.
> Dart is the language flutter (the gui toolkit) is written in.
Well, that's already a strong barrier to its use. When creating a GUI for an app, the toolkit must offer bindings for the language you're using, it's not you that needs to learn a bespoke language just to use the toolkit.
> It can produced mobile, web, or desktop apps from the same codebase...
That's the thing - not really. It seems it can produce a web or a mobile app while will _run_ on the desktop. But mobile and web apps do _not_ work as desktop apps, nor the other way around; the whole UI/UX language is different... even if some companies try to foist mobile apps onto desktop users.