It depends on what you think of as great, but the man has developed two of the biggest software items in the world. One that runs the entire internet, and one that runs most software development.
Just about everything, ever, that any given person has done, would have been done by someone else. Sure we'd be on bsd. Or we'd be using subversion or mercurial. Without Einstein, the same truths about physics exist and would eventually be discovered.
I don't know how you define great, But in terms of good for society, and breadth of impact, I have trouble imagining anyone who's done more in recent times