If Apple offered Metal alongside third-party APIs like Vulkan, I wouldn't be so quick to draw conclusions. However, they made the move to muscle out open graphics APIs from their desktop OS, and the only assumption I'm left to make is that they're directing people onto their own proprietary API in an attempt to lock down their developers and users. Again, if Metal were available on other platforms, or had actual competitors in the space, I wouldn't be so harsh. Until that is the case, it's the same as Apple's App Store: an undeniable walled garden. You can disagree with me, you can cite dissenting opinions if you want, but it's no different than the rest of Apple's product strategy.

When did "walled garden" become a taboo word?

It is not an impossibility, we simply don't know. Just flagging comments _you_ find tiresome is not good for the discussion.