He was being realistic. Let us exclude all peaceful uses of nuclear energy for the moment. After Hiroshima and Nagasaki both the US and the Soviet Union entered into an arms race to see who could build the bigger and better bomb. Look up the Cuban Missile Crisis of 1962 and see just how close to a nuclear war the US and the Soviet Union came. The credit for preventing a nuclear war has to go to the Soviets because they were the ones who "backed down".
Let us now move into the 70's when both the US and the Soviet Union signed non proliferation nuclear pacts, an international nuclear monitoring agency was established.in order to control which nations had access to nuclear weapons. The nations which are signatories to these nuclear pacts realise the danger of the weapons and aren't going to use them. It's known as MAD- Mutually Assured Destruction.
The problem is with rogue states who are members of the "nuclear weapons club" and individual groups with their own ideologies and agendas. They simply don't care or are interested in the consequences of launching nuclear weapons.
If one group fires a missile then you can be sure that others will do the same and the consequence is that if people aren't killed immediately, the radiation will kill them over the next several weeks. Dust will sent into the atmosphere blocking out sunlight- nuclear winter- causing plants to die. Most survivors will die of starvation and those who don't will fight over the few resources left. Then there is the radioactive fallout. The radioactive dust will come back to earth in the form of rain or snow thus further poisoning the earth. for thousands of years. Look up the half-life of elements such as Strontium, Cobalt,
What would be left after a nuclear holocaust would be the debris of cities. There would be no electric power plants, water purification plants, etc.We would be right back in the Stone Age.
You may to see films such as :"Dr. Strangelove or How I Learned to Love the Bomb and Stop Worrying", "Threads", documentaries showing the aftermath of Hiroshima and Nagasaki.
When the first atom bomb was successfully tested Oppenheimer quoted a line from the Veda, "I am become Death, The Destroyer of Worlds".