C++ is useful for learning object oriented programming: Describing what really happens in constructors and destructors, the pros/cons of reference counting and how it actually works (using std::unique_ptr)
These are things that most modern languages try to hide/abstract away, but the underlying problems and limitations never go away, but with C++ you'll have a better understanding of why they happen.
However, if you go down the rabbit hole of Template Metaprogramming, I agree with the original post: it takes decades to learn and really only useful exploitable in C++.