ObjC isn’t going anywhere on Apple platforms anytime soon or even late from now.

Yes, it's true that ObjC is pretty firmly entrenched after so many years driving the platforms. But it also doesn't feel like I'm doing myself many favors professionally by investing any further time into that language. For personal stuff, I can do whatever of course. I dunno, maybe I'm wrong; can't say I've researched "How in-demand are ObjC skills in the job market?" these days to speak with any authority on the matter.

and neither is OpenGL

Metal is written in a mix of Objective-C and C++, the only thing Swifty about it are language bindings, so it isn't going anywhere.

Joining the Metal team is well beyond my abilities. I didn't explicitly say it, but I'm an application developer, not a graphics guru. So for me, Metal might technically be composed of some Objective-C, but from my perspective it might as well not be, because it's a project I'll never be equipped to contribute toward. But your point is taken; Objective-C isn't dead, and my choice of words didn't contain proper nuance.