It also could have stopped the Gods from smiting us all, but there's no evidence that it has.
This article[1] is a good start at realizing the costs outweigh the benefits. There's little or no evidence of good caused, but plenty of evidence of harms caused.
There is evidence of that, in fact. There were many serious terrorist attacks in Europe, like in Spain's subway (300 dead) and Frankfurt, in the aftermath of 9/11 and other...uh howmy gonna say this...other stuff, the Spanish terrorist attacks were done by Basque nationalists or such, not Muslims.
So there's your control group, Europe.