Agreed. Most seemingly crazy occurrences in history were not unprecedented.The better you understand the period, the more plausible you realize every major historical event was.
In any case, saying OTL wasn't plausible really doesn't compute because it's the only TL that we know to have actually happened, so it's the only criterion we have to judge the plausibility of an ATL.
Mongol conquests were preceded by thousands of years of nomadic and Turkic expansion setting up khanates that the Mongols would later unite. All of which descended from the Gokturk Khaganate. The Mongols themselves would partially assimilate to the Turkic tribes they conquered. The specific campaigns by Mongol generals seem extraordinarily lucky but when you look at the historical process they represent it's really not that surprising.
Alexander the Great's conquest was preceded by almost a thousand years of Greek trade, piracy, and mercenary service in the Near East from Ancient Egypt to the Ionian Revolt and Xenophon. Alexander conquered a Persian empire that used substantial Greek forces in its military, had substantial regional decentralization and faced unstable dynastic politics. Some claim that Bactria was already a remote region to which Persia had deported Greek prisoners of war, the Achaemenian's "Siberia", which also explaining how quickly the Greeks seem to have set up a power base in Bactria after the conquest.
Ottoman expansion occurred in a context of a Middle East already dominated by Turks and Turkic peoples for multiple centuries. The Islamization and spread of Turco-Persian culture was hundreds of years in the making. Turkish Anatolia itself had previously been at least nominally united while Asia Minor was previously the center of powerful empires. Further, the core territory of the oldest Ottoman state was the territory of Nicaea which had previous successfully restored the Byzantine Empire.
And so on. Of course these are already oversimplified explanations but everything is a part of a larger trend. That being said, I still believe that the Great Man Theory is the main driver of history, as it is the only way I can logically explain the success of Danny DeVito.