Historically that's been held up as an interesting pivotal moment. But I don't think it bears scrutiny. Germany was itching to go to war (as a natural outgrowth of their manifest destiny). That shot was an excuse. They would have found another - the world was in turmoil.
Importantly, _Serbia_ wanted to cause a war that would pit Austria against Russia. One of the parts of https://www.amazon.com/Sleepwalkers-How-Europe-Went-1914/dp/... that struck me was the offhand mention that Serbia was the one country involved that accomplished all of its foreign policy goals as a result of WWI.
I suspect that even if the assassination had failed they would have kept trying other provocations at least until they provoked an Austria-Russia war.
I think that's exactly the point though. That event was the catalyst, but it just as easily could have been any number of other things and it is unpredictable when it will boil over.
