[darklion]

> had already committed to development of Metal before they knew AMD was up to something.

They might have, but it's sort of a moot question. In 2012-2014, Apple was supporting four different graphics processors across their various product lines: Intel, Nvidia, AMD, and Apple A-series chips. Even if AMD had announced Mantle the day before Apple started working on Metal, it would've been a solution that only covered the AMD category, whereas the first version of Metal was capable of running on all of those architectures. [1]

The only alternative to Apple rolling their own would've been Apple working in conjunction with Kronos to develop a next-gen OpenGL. I can't find anything that indicates Apple approached Kronos, so it's hard to say whether Apple decided against it for technical reasons (Apple may have felt it would have been too long a time frame to reach a final form, or the design-by-committee approach would not have resulted in a satisfactory-to-Apple result) or political ones (perhaps there was insufficient appetite for that scope of change until Apple announced it was abandoning OpenGL for Metal).

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[1] https://en.wikipedia.org/wiki/Metal_(API)#Supported_GPUs