> There's already a panoply of CUDA alternatives, and even several CUDA-to-non-Nvidia-GPU alternatives (which aren't supported by the hardware vendors and are in some sense riskier). To my knowledge (this isn't really my space), many of the higher-level frameworks already support these CUDA alternatives.

On paper, yes. But how many of them actually work? Every couple of years AMD puts out a press release saying they're getting serious this time and will fully support their thing, and then a couple of people try it and it doesn't work (or maybe the basic hello world test works, but anything else is too buggy), and they give up.