From a 1960s point of view, it was optimistic to suggest that the Third World War would be as much as 30 years away rather than 20 or 10 or 5. Also, people had shorter life expectancies then because they drank and smoked and polluted so heavily. Look at "The Deadly Years" and its expectation that Kirk at the equivalent of his upper 60s would be more decrepit and senile than William Shatner currently is in his mid-80s. So 30 years seemed like more of a long-term forecast to them than it does to us.
Besides, look at some other predictions of science fiction. Lost in Space in 1965 had us sending colony ships to the stars by 1997. Blade Runner in 1982 predicted we'd have multiple interstellar colonies by 2019. "Space Seed" only posited interplanetary sleeper ships 30 years out, which was relatively conservative.