I absolutely get the point about Scala, and hadn't looked at it from than angle before; can totally see that in a decade.
The difficulty with C++ does ultimately boil down to what you describe. But IME Objective C is an easier language in which to use OO features while retaining the flexibility of C where required - so it doesn't always have to the the case that a multi paradigm language fails. While ObjC is basically C + Objects + dynamic runtime, C++ has lots more stuff in there; I guess that's the kicker.
The difficulty with C++ does ultimately boil down to what you describe. But IME Objective C is an easier language in which to use OO features while retaining the flexibility of C where required - so it doesn't always have to the the case that a multi paradigm language fails. While ObjC is basically C + Objects + dynamic runtime, C++ has lots more stuff in there; I guess that's the kicker.