Earlier quoted context omitted.
Both of which are now practically deaf on mobile. Flutter comes from Google, and while that doesn't guarantee it'll live, at least it has better potential.
They are only dead on the SV HN bubble kind of thing. Dart also comes from Google and if it wasn't for Ad Words team it would be more dead than anything else. Flutter is it very last opportunity to stay relevant. Given that, I rather bet in dead technology with useful programming languages.
Flutter is the last option for Dart to stay relevant sure. You learn a programming language in a couple of days though so it just doesn't matter, and doesn't change the fact that in mobile world C# and C++ are just as irrelevant. I worked for years with C++, but I don't expect to ever again need it in a professional setting.
This is of course just from mobile perspective. Everywhere else Flutter is currently useless, and Qt/C# perhaps a good choice.