Contrary to the dogma of certain politico-economic camps, a monopoly (the actual presence of market power and absence of substitution effect marking an absence of actual competition in some space) can exist without competition being illegal. So, “Any…competitor could even legally re-implement [CUDA] for their hardware” is not a counter-argument to CUDA being the basis for an actual existing monopoly.
It might be an argument that, to the extent that that is the sole basis for the monopoly, the monopoly is unlikely to be a long-term stable condition, but its not a counterargument to it existing.
Then let's not mince words. This behavior is not illegally anticompetitive. Nvidia's advantage is fair, and they only monopolize GPU compute APIs because their competitors literally abandoned their own solutions.
No one said it was. Having a monopoly isn’t illegal in the US at all (leveraging it in certain ways is.)
The claim was that (1) NVidia has a monopoly, and (2) the effect of that monopoly has been consumer devices getting worse for this use in specific, well-defined ways. Legality of NVidia’s actions and fairness of how their market position arose are not particularly relevant.
It might be an argument that, to the extent that that is the sole basis for the monopoly, the monopoly is unlikely to be a long-term stable condition, but its not a counterargument to it existing.