Chingghis Khan’s greatest failure

the decline of the mongol empire hadn't started until half a century after genghis died.

iranian? you mean transoxanian, and actually we can't be sure if it was done by Temujin or Timur. secondly, he was a nomad, it was only natural to do so, irrigated terrain is hostile to the nomad lifestyle.

Only his own children were supposed to have a place at Kuriltai, but they became so numerous that the Kuriltai stopped functioning normally.

Chinggis never went to Iran.

how could falling off a horse be a major failure, he was an aging old man 60 some years, it's hard at this age to ride a horse for anybody

His greatest failure was that he lost his first or initial battle with jamukha (after they went there separate ways). However, this initial failure did lead to much success, since he took this failure and absorbed many positive lessons through it, such as the training of soldiers, cavalry, and the conscious vow not to fail again, which he clearly fulfilled.

The only reason that I believe this is his greatest failure is due to reality that it seems like his only official failure in battle, since after that nor did he loose a battle or get repelled in his conquest to conquer the world.

Another mistake that might be considered here is his decision to kill his half brother when bullied for a carcass. Although I understand he had to show him a lesson I still fail to comprehend if there was really any benefit for him, since he killed one of his own at a time when that was all he had in this world.  Also he decided to take this bold reaction despite the dissatisfaction of hoelun who clearly told him to refrain from any action.