From time to time I hear hot-heads suggesting that we "destroy Islam" by nuking Mecca and Medina in some kind of super-ArcLight. No Kabba or Mecca means no haj which means one of the pillars of Islam is gone, and it all falls down, right?
Um, no. Set aside for now the immorality of killing a few cities worth of people, maybe half of whom aren't our enemies. The plan fails on its own terms anyway.
Remember that Islam is, despite the best efforts of the Saudis, a fairly diverse collection of schools of theology/politics. In fact there are 5 major classical schools, plus a couple of new ones: Qaddafi's variety and the Wahabbi school (which I'm told has a sufficiently different approach to the Hadith from its parent Hanbali to count as a different school). The Sufi aren't counted as a separate school, but their influence is reputed to have been substantial. They tend to focus on inner spiritual qualities rather than the outward rituals.
Since pretty much any respected scholar can make fatwas, you can find vast numbers of them governing the minutia of life; and you can find contradictions among them even within a particular school. To make matters even more flexible, some scholars produce fatwas designed to achieve some result, rather than on the basis of clear scholarship. (one reason for the contradictions mentioned above)
The haj is only mandatory for those able to make the journey. Illness and poverty obviously influence ability, and so presumably does the danger of the journey. If Mecca is a radioactive hole, I assume it is too difficult to go there.
The day of ijtihad was declared over long ago. But if Mecca disappeared, certainly one of the schools would reopen ijtihad and re-evaluate the pillars of Islam in the light of recent events; and if one does, all will have to. You might speculate about possible Sufi influence on the results, since the literal application of the Koran wouldn't seem to apply anymore. I'm certain that the scholars would come up with some clever re-interpretation that would keep them in business if they had to. They can say things along the line of:
Five times a day, pray towards where Mecca was. (A no-brainer)
The haj was only mandatory if you were able to do it. Since no one is able to do it any more, it is obviously not mandatory. (Seems fairly benign and irrefutable)
Mecca is gone, so the end is near, so we need to be found as devoted fighters for God! The new pillar is jihad! (Sounds like bad news)
Where is Mecca? It can't be just the city, which kept changing over the years, (and no doubt became polluted in the process), but the place of Muhammad's obedience and Abraham's obedience. And if one place of obedience is gone, we can obey in another. We can make our pilgrimage to a nearby place from which we can see where Mecca was, or (if the Sufis have enough influence) in any place where one of God's prophets has been.
God sent the stone of the Kabba to Earth, and He's entitled to remove it if He doesn't like the way we've been treating it, right? And in any case the stone isn't supposed to be all that critical, no matter what the folklore says. (True. It isn't.)
But I don't believe they would have to work hard at it. At least one, and probably many, Muslims visiting the outskirts of the cratered zone would find a black rock, and inspired by dreams of glory or else just dreams would declare this to be the miraculously preserved stone of the Kabba. If some local scholar is on the ball, he'll celebrate the first find and cut off further searching (violently if necessary); and the New Kabba will rise in New Mecca. If he's not on the ball, the scholars will face a number of contenders and a more complex problem. The clever thing to do would be to recognize all of them in some sense and stash them all in the same building, but that may not be politically possible. As a fall-back position they could recognize all of them in some sense, and urge their distribution around the world into many Mecca's, which God mercifully provided because the first one was getting too crowded.
My point is that even if Mecca vanished, it takes little creativity to justify a new Mecca, or many new ones. Sufi influence might make the new Mecca a local or symbolic inner one; and fake relics would easily provide a center for a New Mecca. And in any event, people who would abandon Islam if Mecca vanished are probably not the dedicated sort who would be our enemies anyhow.
And no matter whether they take the position that God was angry and withdrew Mecca/Kabba, or that God protected its essentials from the devices of the devil--we wind up in the role of the devil. It would automatically become a fundamental Muslim doctrine that Americans were devils, and we'd have a billion devoted enemies instead of a few hundred million. Forever.