How far are western intellectuals prepared to take civilizational self-loathing?

In The Decline And Fall Of The Roman Empire, Gibbon called the Battle of Poitiers in 732 “the high-water point of the Muslim tide in western Europe“. Had they won, they’d have pressed on to Paris and London and beyond. But they didn’t win – which causes David Levering Lewis, the American historian and author of God’s Crucible: Islam And The Making Of Europe, 570-1215, much regret:

Lewis sets out to show that the failure of what he calls “the jihad east of the Pyrenees” is “one of the most significant losses in world history.” He argues that the Frankish defeat of the Islamic invaders at Poitiers in 732 and the subsequent poetic glorification of Roland’s sacrifice to cover Charlemagne’s retreat from his own incursion into Spain were “pivotal moments in the creation of an economically retarded, balkanized and fratricidal Europe that, by defining itself in opposition to Islam, made virtues out of hereditary aristocracy, persecutory religious intolerance, cultural particularism and perpetual war . . . ‘winning’ at Poitiers actually meant that the economic, scientific and cultural levels that Europeans attained in the 13th century could almost certainly have been achieved more than three centuries earlier had they been included in the Muslim world empire.”

As Tim Rutten, the Los Angeles Times reviewer, remarks:

In other words, the West would be better off if it had been incorporated into an all-conquering Islamic empire in the early Middle Ages.
OK.
Still, it’s fair to wonder why, if that’s true, the West ended up with the Renaissance, the Enlightenment, the Industrial Revolution and the Scientific Revolution and the Islamic world got chronic underdevelopment, a pervasive religious obscurantism, Al Qaeda and the trust fund states of the Arabian peninsula?