As I recall, Einstein himself once said that someone would have developed Special Relativity in short order if he had not, but he didn't believe anyone would have developed General Relativity at that time. Also as I recall, when someone asked Minkowski if it was true that only three people in the world understood General Relativity, he stared into space for a moment until the question was repeated, and then he replied, "I was just trying to think who the other fellow was." No doubt someone would have developed it eventually, but it might have taken a while.