It was a huge waste of money, the US was always a paranoid state which was extraordinarily secure. This is precisely what Eisenhower warned about, the rise of the MIC which has its own self-sustaining logic.
I partly feel that the real reason it was a waste of money is that after it was finished, the country was too nervous due to Francis Gary Powers' U-2 being shot down to actually try using the SR-71 for its original purpose of flying over the Soviet Union, meaning they were using a very expensive plane for the missions.
Ironically enough, the much cheaper but less impressive U-2 is still in use today.
