Mutually assured destruction has worked fine for 80 years, why stop now?

While it's an extremely hard problem, it's not completely unsolvable because there are a finite number of GPUs on the planet capable of doing frontier model development, and they use a lot of power. The vast majority of them could be tracked. China and the US could agree to joint monitoring and they could each verify what ~95% of the other's compute power was up to.

A lab of researchers without compute isn't going to accomplish much, there is a huge physical footprint unlike bioweapons research. But, the political aspect is unsolved.