Using the same logic (important diplomats dead or retired or disgraced) you could say that firing Bismark caused the war.

If the top diplomats had managed to avert the Serbia crisis then the war would have been delayed until the next crisis.

The fundamental problem is Europe had built itself a civilization-strattling war machine with no "abort" button and put the start button on a hair trigger (the interlocking treaty system).

Yes, a common view at the time was that the only de-escalation possible would have been through workers cooperation, which is why so much of the Marxist flank of the SFIO argued in favour of pacifism: to buy time for the workers to unite.

Which probably wouldn't have worked, but maybe it could have been a much, much smaller conflict.