That would be like trying to get every country to agree to give up nukes.

Or agree on finding ways to promote peaceful use of nuclear energy. This has been done, there are thousands of people working on it around the globe and 180+ member states of the IAEA. It's not easy, there have been close calls.

And cooperating interntionally to buy ourselves time to find ways to develop this "last invention" is a way that will do good for humanity seems to be on a similar level.

Or stop making more, and testing more, which we got the biggest countries to do, at least for a time.