If you mean to say that WWI could have been avoided if FF and his wife wouldn't have been killed, I don't think so. Wilhelm was preparing for to long for this war, he just had a good excuse with the assasination. But you're right, the 20th century has been terrible. Hopefully my children will be born in a free world. I'm lucky to grow up most of my life in peace, but looking at my family: war, camps, prison,...
But what do you mean with "class" has died that day? If you mean western culture, I don't think it has. Even through everything the world has been through during the last century, it managed to produce people as Gabriel Garcia Marquez, Solznjetsin, Marcel Pagnol, Isabel Allende, Orlando Figes, Victoria Holt...(I'm just looking around me at my bookcases

), the Beatles, Steven Spielberg, ... and so on , and not to mention this amazing website we're posting on.
I think we tend to romantize the past too much. We look at pictures from hose beautiful days forgetting that only a select few were actually enjoying it, we forget the immense poverty, the colonization, women and children dying (I'm talking about the west in those days, because most of this sh** is still reality in the third world and in smaller amounts in the west)