My point isn't to talk about the various tradeoffs between Metal and Vulkan. Ignore that Metal exists for now. My point is that by not supporting Vulkan, Apple isn't supporting the lowest common-denominator API that everyone else supports. So that leads to people having to make a Vulkan implementation, and then also a Metal implementation if they want to support macOS. Regardless of the benefits and drawbacks of each, there isn't a way to support every platform with one API. And Apple causes a lot of strife because of that.

I don't have a problem with Windows having DirectX12 in addition to Vulkan. Or even that vendors typically add new features to DirectX12 first, with Vulkan getting them later. This is because you can still write a Vulkan program, and have it run on Windows. Apple only supporting Metal breaks "write once, run on any platform". It's one thing for Apple to have Metal as a better supported/approved API that they can make the best apps with. It's another to lack Vulkan support altogether. Because Apple aren't just trying to get the highest Metal adoption. They could easily do that with AppStore restrictions or something, or just accepting that UIKit using Metal is good enough adoption. Lack of Vulkan has knock on effects on every other platform, because now engines have to do a ton more work, and likely have more bugs.