How could Rome/Byzantium afford to transfer away from the Heavy Infantry core legions of the Republic and Empire to the Hun/Mongol like Cavalry armies of Byzantium
Infantry were cheap and easy to train and produce due to the impressive roman logistics chain while horses and riders were rare as they were expensive and italy wasn't horse country.
so how did Byzantium which is smaller and had less resources than rome afford to train, house, feed, and equip such large primarily horse armies