Don’t buy it.
Merchants that become more successful do not generally morph into conquering armies. At least I can’t think of any examples. They expand their merchant activities, instead. Merchants are more likely to turn warlike when their trading activities are blocked.
IMO the Roman and Persian Empires had fought each other into a comprehensive state of utter exhaustion, something like some claim would have happened had we stayed out of WWII and allowed the Nazis and Soviets to destroy each other. (I’m not one of these people.)
So I think there is no question there was a major power vacuum in the Middle East. The question is why the Arabs, who had absolutely no tradition of conquest, suddenly became conquerors for a century. And then stopped again.
Again, IMO, the answer is the power of a new religious idea that fitted exactly into the pre-existing ideas of the Arabs.
I don't believe the Soviets could have struck back. the Germans were just too strong. They swept straight into the European half of Russia with next to no issue. Only American arms and support kept Russia propped up.
If that had not been there and if (I know, many "ifs") the Japanese had hit Russia in the back in Syberi, the Nazis would have ridden rough-shod straight into Central Asia.
What would have happened then is the same with any over-stretched Empire, especially one that over-stretches so quickly -- it would have broken.