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.
Even 'forced' Christianity wouldn't do it if they really believe that in the end Allah will save them.
The exercise would be one of futility.
Just as most Christians can't be forced to be muslims. Most muslims will not be forced to be Christians.
It's better to let them stay bewildered and lost for as many generations as necessary then to create an even greater hatred of Christians, which would give them a stronger reason to hold to their faith and allow them to recover sooner.
As you say, the devastation must monumental and complete in order for them to even question Allah and his teachings.