Metal was released soon after and then as a finished "product" in iOS. It likely has had more than a year in development inside Apple before the release too.
You make it almost sound like they coughed up Metal in the few months after Mantle was shown.
If they want, they can switch. In the long run Metal either: has no future; or must be opened; or will remain a proprietary tech with little advantage and recurring dev costs for both Apple and app devs.
With the existing competition, I personally don't think it will be both opened AND widely adopted. Probably already too late. So either you really want to get on Apple computers now, or the tech is, IMO, basically irrelevant.
Whereas OpenCL is, yes, laborious, but still relevant in the open world. And Vulkan is also used across different vendors.
Apple can get away with their current "my way or the highway" attitude solely because of their user base actually not negligible, and somehow "rich". Devs are not liking the platform a lot, and I phrased that carefully to be kind. Most of their "pro" product have been discontinued or have become laughable. The new Mac Pro is an interesting counter-example, but for a very niche market; which means that even with their pile of cash, they choose to concentrate instead of attempting to conquer new markets.
Given they sell basically luxury products, it is doubtful this will remain like that forever. They might transition their model, but I'm not sure toward what. Everybody talks all the time about services, but that's clearly not what vendors of gadgets are relying on for now. As for artificially monopolistic or quasi-monopolistic "app stores" and/or "clouds", I do not even consider that a "service", and they obviously rely completely on the consumable gadget model.
Metal was released soon after and then as a finished "product" in iOS. It likely has had more than a year in development inside Apple before the release too.
You make it almost sound like they coughed up Metal in the few months after Mantle was shown.