WI: Kushan Empire conquers Parthians, cerca 127-150 CE


Kanishka the Great, ruler of the Kushan Empire, oversaw the realm's greatest expansion during the first half of the 2nd century CE. He secured the entirety of the Indus Valley and expanded across northern India perhaps a far as Pataliputra on the Ganges. At the beginning of his reign his Parthian neighbors to the west were split into two kingdoms, one based in Mesopotamia under Osroes I and another based on the Iranian plateau under Vologases III. OTL the western kingdom was conquered by Vologases in 129 CE, but ITTL Vologases is defeated soundly and his kingdom is left badly weakened. Following this defeat Kanishka, after allying with the nomadic Alans, invades the eastern kingdom and takes control of the Iranian plateau. A decade of rebellions follows, leaving Kanishka capable of securing the Indus Valley, but not enough to expand down the Ganges nor to contest dominance of Kashgar and the rest of the Tarim Basin cities with China.

My question is about the stability of this situation. At first glance it seems that the external threats, other than nomadic incursions, are limited. The was no great empire in northern India like the Maurya, China had no interest in territorial control west of Kashgar, and if the western Parthian kingdom in Mesopotamia can be kept as a buffer state, perhaps peace with Rome can be maintained. Like OTL, the Kushans will be rich from the control of Silk Road routes, so even the threat of steppe tribes can be mitigated by paying tribute. Thoughts?
Another aspect to this timeline is the westward expansion of Buddhism. Kanishka himself was a fervent Buddhist who promoted Greco-Buddhist artforms and built massive stupas. OTL there also existed Parthian nobles who were Buddhist, ITTL even more nobles will convert, increasing the pool of potential missionaries. If the Mesopotamian Parthian kingdom becomes a true client of the Kushan Empire, Buddhism might well become its predominant religion, which could have knock-on effects upon the development of Christian theology in the Levant and Asia Minor.