I wonder to what degree the spread of Islam was on the influx of fresh DNA / outbreeding via the captured concubines. The first generation took over by surprise and zeal - the second through training by experienced warriors and without inbreeding depression of their intelligence.
Their "Golden Age" lasted only as long as their expansion. Once they no longer had an influx of loot and slaves who knew how to run a civilization, the Islamic world stagnated.
Polygamy fed their wars of expansion. A man who cannot afford the bride price to get a Muslim wife, will be quite a zealous warrior if he knows that his only hope of getting laid is to capture a woman of his own.