Questionable, or I'd say depends on your taste. In mobile sphere C++ does just fine, especially when combined with Qt and QML. And performance still matters most of the time, whether we are talking about embedded or server systems.

If you are going to use Qt/QML you might as well move to C#. C# will always be more performant than JavaScript (QML is based on JS). Plus the bloat introduced by Qt (signals and slots etc) makes it just as slow as C# without any of its benefits.

What is that "always" based on? In Qt QML is needed for the interface only. All application logic can be written in C++ so performance is not an issue.