It's true that an invasion of the US mainland was never in the cards, given the situation during the cold war. Pretty much nobody was seriously worried about that. What NATO was very worried about was a Soviet invasion of Western Europe, which is a very reasonable worry considering the number and detail of Soviet plans for doing just that.
For their part, the Soviets were very worried about a combined NATO invasion, which seems quite reasonable considering that Germany alone, while also fighting/occupying most of the rest of Europe, managed to get to the suburbs of Moscow, and kill millions of Russians along the way.
Now if the Soviets did manage to invade and conquer Western Europe and maintain control over it, then they might be in a rather better position to consider direct attacks against the US. Which is a good part of the reason why the US invested so much in keeping them out of Western Europe.
For their part, the Soviets were very worried about a combined NATO invasion, which seems quite reasonable considering that Germany alone, while also fighting/occupying most of the rest of Europe, managed to get to the suburbs of Moscow, and kill millions of Russians along the way.
Now if the Soviets did manage to invade and conquer Western Europe and maintain control over it, then they might be in a rather better position to consider direct attacks against the US. Which is a good part of the reason why the US invested so much in keeping them out of Western Europe.