Metal Shading Language is a C++ dialect. HLSL is as much of a C dialect as GLSL is. And I would argue that C++ provides no benefits over C for shading languages.
Moved but remained proprietary. So the problem is not solved. Language shouldn't be proprietary to begin with. It's an obsolete and invalid practice from the last century.
Apparently not, given the monthly occurrence of FOSS projects that decide to go dual license, or even back to proprietary, because this century practice doesn't cover their existence and life responsibilities.
For languages? That's a dead end really. That's why whatever Apple will produce will also be dead, as long it's proprietary. Apple is stuck in that mentality and has hard time getting out of it.
I prefer GLSL to Metal Shading Language.