Any change, no matter how small, has geometrically increasing ripple effects. And to even suggest that World War I and all the horror that flowed directly from it (Including, but not limited to, the Spanish Flu pandemic, the Russian revolution, Stalin's regime, the rise of Communism in Europe, the second world war, the Chinese revolution, the rise of Communism in Asia, the cold war, the nuclear arms race, the rise of the military-industrial complex, the wars in Korea, Viet Nam and Central America, the Kim dynasty in North Korea, the occupation of Afghanistan, the ethnic wars in the Balkan, and ultimately even the war on terrorism, all with a total body count I can't even begin to estimate) were a price worth paying for the progress that followed from it - a questionable notion in any case - seems very cynical to me.
But here's the thing: You wouldn't know what would happen. Leaving something like that up to chance is just something I could not live with.
I am a personal believer that the only way to improve is to look to the future and plan for better. I can do this because of the mistakes that have already happened.
In the case of changing a major political event like World War 1, you wouldn't know what would happen, You would just assume that "Everything will be better" (again, selfishly). What if it is not? What if it turns out to be even worse? Would that not make you the biggest war criminal ever? Sure, as said, there is a chance it could turn out for the better... But leaving something that big to chance isn't something I don't think I should personally make.
Kudos to you if you have the fortitude to attempt and accept responsibility for such a thing. But that fact everyone assumes that it would be better, is false. There are always an equal (of infinite) amount of chances that it could turn out for the worse.
And if it turns out worse, and you "write yourself out of existence", everyone in the world has to live with YOUR choice, because you assumed it would be better.