We want to make sure people believe in the deterrent. If people start to think they could take out all of our nukes somehow that would be dangerous.
We have been in this very position before, in 1948, when we were the only people with nukes. We didn't first strike then.
The other difference is that literally everything has changed in the past 76 years. But sure, maybe if we dig up Truman, Stalin, Chiang Kai-shek, Mao, and Churchill, and put them and their cabinets back in charge, it might work out.