degradation of Greek cavalry in Hellenistic era

[Sean Manning]

I don't know if the sucessors thought cavalry was inferior (even the greek polis' used heavy cavalry when they could get them) it was just that a lot of the sucessor generals tried to emulate Alexander's style but didn't quit manage it. Where he would break through and then turn to the roll up the enemy flank they would keep going until they were far away from the rest of the army and their horses were exhausted.

I am not sure you are being 100% fair. How often did Alexander face large armies equipped and trained as well as the armies fielded by the Diadochi? Lets be honest, beyond the Greek mercenaries and porbably the "Immortals" the Persian Empire deployed infantry that was little better than rabble compared to Alexander.

That's a strong statement. Other than the battles against Alexander, how much do we know about the effectiveness of Darius's troops? Persian infantry fled quickly at the Granicus, had some successes and some failures at Issos, and we can't tell how they fought at Gaugamela because our sources don't discuss it. They also gave Alexander a hard time in sieges and at the Persian Gates. Persian troops (like Gauls in Caesar's day) could have been fairly good soldiers without being able to beat a smaller army of veterans under brilliant leaders.