I had to go with the archduke surviving. Maybe without his death at that particular time, WWI would have butterflied away into several smaller regional conflicts, with much lower loss of life, the avoidance of the rise of the Communists in Russia and the Nazis in Germany, and the 20th looking a lot better.
Conversly, however, WWI could have started over something else and been just as bad as OTL, or even worse. Given the alliance system of the time, I know a lot of people think WWI was unavoidable.