Metal shaders != Metal API. Metal API is Obj-C. Metal shaders are written in MSL (Metal Shading Language). C++ is still not involved, though MSL is admittedly based on C++.
"The Metal programming language is a C++14-based Specification with extensions and
restrictions. Refer to the C++14 Specification (also known as the ISO/IEC JTC1/SC22/WG21
N4431 Language Specification) for a detailed description of the language grammar.
This section and its subsections describe the modifications and restrictions to the C++14
language supported in Metal."
Also, the MSL and Objective-C compilers are built on top of LLVM, written in C++.
I don't know what point you're trying to make. MSL doesn't link with the rest of your program. No matter what language you use for your program, the shaders are written in MSL. For my program if I use Rust or C++ or Obj-C or Swift or a microscopic magnet and a very careful hand, I write my Metal shaders in MSL.
The fact that Metal itself requires the existence of C++ really has nothing to do with this topic though, which is whether there's any downside to writing your app in a language that Apple doesn't ship a compiler for. The fact that Metal shaders are written in a language based on C++ is irrelevant to this question, because it doesn't matter what language you pick for the rest of your app as Metal shaders don't link with your app. Swift has zero integration with C++ and yet I can write a Metal app using Swift, because the fact that the shaders are written in MSL has no impact on the fact that the app uses Swift.
And the language for writing drivers in is even more irrelevant because we're not talking about writing drivers. You can't write drivers in Swift either.