Hardcore Muslims are required to visit Mecca once in their life. If the rock is destroyed, their requirement can't be fulfilled. If we defile their corpses with some unclean pig blood, they can't reach paradise. That is their weakness.
I am not sure if you profess a Faith, but I find that belief to be rather naive in context of my understanding of faith in general. Islam and Cristianity may not be comparable but I guarentee Faith is not so easily shaken. If a Rock is destroyed, their Faith will not collapse. Someone will look within the Koran and find something to pull out that will validate their continued existance. Passage will be found to claim heaven is not lost to them. In the meantime until that is done, the immediate action would be a call for vengenace.