You wouldn't have to replace Islam with Christianity, if they were crushed and felt that Allah would not protect them.
They would abandon it themselves.
They would not likely abandon their religion, even if crushed. You do not understand the abject fatalism of the mussulmans. They will go inert and lethargic for a time, perhaps a generation, perhaps several, but they will not give up their religion just because they failed to conquer. And a new Fever will eventually sweep through them again and the cycle will peak once again with death with destruction throughout the world. This is the history and future of Islam.