The question for today's Europe is whether the primary identity of their fastest-growing demographic is Muslim or Belgian, Muslim or Dutch, Muslim or French.
That's where civilizational confidence comes in: if "Dutchness" or "Frenchness" seems a weak attenuated thing, then the stronger identity will prevail. One notes the similarities between revolutionary America and contemporary Europe: the United Empire Loyalists were older and wealthier; the rebels were younger and poorer. In the end, the former simply lacked the latter's strength of will.
My own view is that Steyn greatly underestimates the confidence of Western culture (and probably Belgian/Dutch/French culture, though I know a lot less about these specifically) because the West has so few self-conscious spokesmen. In fact, though, the shortage of spokesmen is a sign of a culture so confident that it doesn't bother trying to win converts. Western culture, beautiful, fun, and smart, wins converts and undermines the competition simply by existing.
For most of the 20th-century, socialists had hyper-confident advocates, but they lost anyway - in large part because capitalism is simply better-looking. I say the same thing will happen in the 21st-century competition between traditional Islam and Western culture. Islamists have an army of fiery spokesmen. But the West has Paris.