Some coincide it with the degradation occuring after the rise of Islam.
I’ve seen that notion. An equally reasonable one, IMO, is that Islam was able to explode out of Arabia the way it did largely because the classical civilization was already tottering. IOW, Islam was a consequence more than a cause.
It is certainly a fact that the Eastern Roman Empire and Persia had fought an utterly exhausting war for decades, really centuries. By the time Islam showed up, neither was able to put up much of a fight.
Kind of the strategy some think we should have use in WWII. Let the Nazis and Commies wear each other out. :)