Gonna call bull on the Alexander part. He died unexpectedly at 32 and no obvious heir or designsted successor. His only legitimate child was still in the womb at the time of his death and his lieutenants recognized their only chance to seize power was to fight for it.

Absolutely was not Alexanders intent for his empire to be ripped apart after his death or he wouldn't have spent so much time conquering. As far as historians can tell he conquered all he did with the goal of making an empire that could not possibly have a rival strong enough to challenge it. And in those times, people assumed that means they would continue forever, as they had never seen am empire exist long enough in those days to stagnate and implode inward thanks to political infighting.