Part of it is the software world's fault as well. Almost the entire open source machine learning ecosystem is written for NVIDIA's CUDA and nothing else.

And almost all competent non-CUDA platforms (e.g. Google TPUs, Tesla's secret in-house hardware) haven't been open-sourced, or even sold to consumers, which further enables the NVIDIA monopoly.

It's a duopoly, and unless you've got evidence that AMD and Nvidia are colluding to hurt potential competitors there's nothing illegal about it.

It’s really hard to call a duopoly much better than a monopoly. Sure you can probably identify points, but neither outcome is anywhere near efficient. I don’t know that it should necessarily be illegal. But it is market concentration.

A duopoly is absolutely much better than a monopoly. You've got competition pushing you improve.

but by definition isn't a duopoly not competing with each other? I might be wrong, but I remember it being about the two companies making deals with each other to keep both prices high and not innovate or improve, like ISPs in the US

No, that is not a characteristic of a duopoly and in many cases is illegal.

Oh it’s illegal so they won’t do it or have a zillion ways to work around it while being technically legal.

Anticonsumer practices really need a test of the impact on consumers, similar to how hiring practices are tested. There is a give and take in most broadband suppliers, for example, where there is no written agreement to not compete but they don't compete.