I'll mention again the nuclear analogy. It is, believe it or not, possible for great powers, and even adversary great powers, to agree to limit the development and proliferation of dangerous technologies.
> The main secret is out of the bag.
This is not something you can do in a shed with a handful of GPUs just because you know "the main secret". To build something like Mythos you need tens of billions of dollars, massive amounts of power, enormous buildings filled with specialized bleeding edge computer chips that are made by (optimistically) a handful of companies with deep government ties. You need free access to all the intellectual property that humans have created and posted openly on the internet. You need all of this at each step, and to take each next step you (or somebody) needs to have taken the previous one.
For now, there are a million ways for a government to pump the brakes on this cycle.