They believe, sincerely, that if the USA launched rockets at Mecca, Allah would swat them away like flies, or turn them around to hit New York or Washington.
But you post does raise fascinating questions. The conventional wisdom is that if we destroyed Mecca 10 billion Muslims would turn into an avenging army. But would they? Or would, as you suggest, they take it as proof that Mohammad was a fraud?
Horrible to have to ask questions like this but that's where we're at.
It's a hell of a question, ain't it? I think a % of Muslims would commit mass group suicide, ala Jonestown. A % would go totally postal, berserk, and would need to be detroyed like mad dogs. A % would to into catatonic shock. And a % would engage in the largest mass-conversion in history, signing on with the "winning" God, after allah was proven a fake.