Going away from C++ may encourage new contributions but may turn away current contributors just as well.
KDE is a mature project and has to be created using mature tools. The only new language coming close to C++ in this aspect is Rust. It is also one of the few that has actually better language design and doesn’t just benefit from being new.
Rust is a great language, but it has a learning curve steeper than even that of C++, which can also prevent contributions.
Introducing Rust will also need infrastructure for mixing languages, initially making build systems even more complicated, and it will definitely introduce bugs of its own.
With how much improvements new C++ standards see, how good modern static analyzers are, and how much ugly things Qt and KDE frameworks can hide, this is a lot of trouble for not so much in return.
It would be great to adopt Rust in core parts of KDE like KIO, KAuth, maybe KWin. It has great potential there, but doing it will be difficult. While for most apps, it doesn’t make much sense.