I agree with some and disagree with other.The land stretch from the Proto Indo-Iranian homeland near the Volga Basin and Southern Russia to the fertile region of China is incredibly hostile and the possibility that a group will make it through with numbers and fertility rates to make any change is very very small. Cold grassland, cold desert, hot desert, cold forests, etc, you have them all, en-route.
However one possibility of an Indo-Iranian China could arise from a migration Anti-clockwise from the North of the Caspian sea and they manage to pull off an exceptionally good food production capacity. The route could be from Volga Basin (possible homeland)-Caucasus (or via Balkans)- Turkey- Iran- India- Burma- China. Previous point holds good and they need an exceptional food production and management capacity, to use this route and create a predominantly Indo-Iranian China.
Forget the route via the steppes, Siberia and the deserts. They will probably die off on the way.
There is not enough proof in my view to argue that the Xiongnu were proto-Mongollic or Turkic, it is more clear that the nearby Donghu to the east of the Xiongnu were the group that we may say were predecessors to at least the majority of the peoples we come to call Mongols, Tartar, Turks etc... The Donghu preceding the Xianbei and they preceding the first Mongollic realm in the Rouran Khaganate which included a vast assortment of these peoples termed 'Altaic.' The Xiongnu on the other hand were either some kind of group now more rare today such as the Yenisei or were Indo-European of an unknown and lost variety, likely a mixture of both.The Han dynasty Chinese where familiar with the Wusun, who were Indo-European nomads. They were conquered by the Xiongnu though, who were a proto Mongolic/Turkic/etc. confederation. To conquer China, you would have to be at least as powerful as the Xiongnu or China needs to suffer one hell of an stability crisis.
Some evidence suggests that at least some Xiongnu tribes were Yeniseian and they strongly influenced the Turko-Mongolic peoples in that area. For instance, the word "khan" may have come from a Yeniseian language at an early date as "*qeh" (great) as it has a cognate in the Athabaskan (a split from the Yeniseians) ending "*-ka" which signifies seniority, wealth or the status of chief in numerous Athabaskan languages. We can also determine mythological parallels between suggested Yeniseian myth/folklore which was borrowed in Turko-Mongolic and Athabaskan myth/folklore.The Wusun and the Yuezhi were forced to move west being defeated by the Xiongnu who certainly were the proto Mongols.
Maybe if the Shang dynasty was hit by a plague killing 50% of population.
Or the Aryans just had some inherent trait that would make them assimilate the Han. THey did assimialte most Dravidians in China
I think an interesting fusion of cultures may emerge.
How to combine Indo Aryan religion with Shang Chinese one?
Well we don't know exactly, post-Roman England certainly saw a demographic decline in the 5th and early 6th century, decline to maybe 1/2 or even 1/3 of the peak population, but compared to the late Roman population the decline wasn't that large.For that matter, Roman Britain didn't lose 50 percent of it's population when the Anglo-Saxons arrived nor did Europe in general see a mass die off when the Indo-Europeans showed up.
You saw replacement events later on too, Turks definitely had a big impact on Anatolia and Central Asia, same goes for Slavs and Anglo-Saxons and Ibeiran colonies specifically too parallel those kind of dominance on the Y-DNA side.A completely different approach from later Indo-European and Turko-Mongol steppe societies, who only had minor genetic impacts on conquered populations.