The hetairoi were around until the defead of Antiochos VII at the battle of Ecbatana in 129 BC , then he Seleucid military was broken beyond repair the hetairoi most likely ceased to exist after that disaster
The late reform is in 222BC, far too early to pre-empt that sort of thing.
Note that parthian saddle were made so that the guy on it could fix his chest and legs and "be one" with their horse. It was not stirrups (better for balance, allowing the wielder to be standing while giving blows) but it was enought to hold the two handed kontos.
Try to handle a two handed kontos with nothing to hold to, and you"ll understand why they managed to find a way to hold to their horses with specific saddles that gives them more grip.
The charge with a kontos is know back in those years to be devastating. Macedonians used it, Skythian used it and Parthian used it.
It is not couched lances, but couched lances is a gameplay myth based on a detail that people saw and said "woooooow" and errected to be the "ultimate weapon".
Stirrups are for standing and being balanced while doing so. Their influence in "couched" charge is dubious at best, and mankind didn't wait for stirrups to invent devastating charges.
The same can be said for medieval times : couched charges was used mainly in tournement. In war and battle, it was used when possible and it was not often the case. A frontal charge is not a good thing unless your enemy is not in rank.
Cavalry was better used by flanking and chasing, even in "medieval" times.
But, sadly, myths are hard to break.
Stirrups give side-to-side stability; useful in a standing melee, and to give a more stable platform for archery.
tell that to the seleukids who always had a strong cavalry arm. If there should be something done about seleukid hetairoi i would suggest giving them an armour upgrade. It is mentioned that seleukid hetairoi where almost as heavily armoured as the kataphraktoi during antiochus III reign