OpenGL is limited to 4.1 on Apple platforms if I am not mistaken, and will not get updated ever. So you will run into missing features eventually on some platforms, e.g. SSBOs are really nice but you can't them use on Apple devices.
For me, a much stronger argument against OpenGL is that is requires a global state. This often leads to bad design, is miserable to multi-thread, and is rather tedious to port to Vulkan. I have spent several years fighting with multi-threading applications doing all sort of things in OpenGL and it is not great. Yes, Vulkan is extremely painful to set-up and you have to think about a million things you might not really care about directly, but honestly, I would also recommend against starting anything serious in OpenGL. To get your hands wet maybe, as with a couple lines you can get something running..
I think if you're trapped in the Apple ecosystem then Metal is your only production-quality option. But beginner-level learning material for it is quite sparse, so it's better to transition from OpenGL to Metal rather than learning Metal from scratch. (If you're an experienced graphics dev, you should be able to follow up on Metal by just reading their example source code)
There are a few nice books, they are a bit out of date though.
MoltenVK is pretty good I've been told
Nope, the limitations are just too huge for actual usage. Heard Godot Engine tried to use MoltenVK for macOS but experienced too many issues so they just developed a Metal backend.
The fundamental problem is Metal 3 is just too high-level to be able to emulate all of Vulkan's behavior. The new Metal 4 API (which is more low level and similar to Vulkan in many ways) might have improved things recently, but sadly MoltenVK hasn't been rewritten to this new API yet.
Worse than OpenGL though? Haven't had the time to really look into it, probably native Metal is better, yes, but either way it might be easier to port/adapt from Vulkan than from OpenGL (due to similar concepts, no global state, etc)
Yes, hence there is now a new layer, KosmicKrisp.