All true, but the Successors were competing against their peers, and as SlyDessertFox notes, Alexander's family was not killed immediately; this was a gradual process of radicalization and burgeoning ambitions that led there. Alexander is in a different category, as he is the rightful king, and world-conqueror. Seleucus might have resented the notion of being subject to Antigonus, but both were content to follow Alexander. Of course, Alexander's empire will face the stresses of any state extending from the Mediterranean to the Indus, but as long as he lives and does not descend into madness, his generals will likely remain loyal, and the military superiority of Alexander's army against any potential enemy in the Med or Arabia is enormous. A critical issue IMO would be the transition between the generation of the men he conquered Persia with, and whoever their successors are to be; many sons of the old guard will be among them, but many new men, including Persians, as well. If the old guard felt they were being pushed aside/replaced, this could become dangerous for Alexander. Augustus had Agrippa, but Alexander has a dozen Agrippas to keep happy.