Not sure whether I understood your statement correctly, but you can use Swift for using metal API already. Swift is not the bottleneck for TensorFlow, PyTorch like open-source machine learning library appearing for macOS based on metal, it is the closed nature of mac ecosystem itself.

Look at the flak Google, Facebook get regarding these projects in-spite of them being open-source. If an equivalent metal (only) based open-source machine learning of this scale did pop up with Apple's help; it may not go well with the ML/DL community as sharing & collaboration is valued high here. Apple has even started to publishing open ML papers to attract talent.

Then again, Apple doesn't get as much scrutiny as Google/Facebook does with the tech community, take a look at any HN thread on GoLang/Swift; the discussion on the former goes to the company rather than the programming language as in the case of latter in-spite of both being open-source.