I suspect that it may be without a concerted military defeat at the hands of a conquering competing religion.
I believe that the reason that Islam is so unchanging and so successful is that it turns all of a man’s vices in to virtues and consecrates them as laws of Allah.
One of the primary vices that Islam holds high is a man’s desire to have many women. Islam holds that a man may have multiple wives and that if he desires to have other wives than those he has he can divorce his unwanted wife with only 3 repetitions of the phrase “I divorce thee”.
Islam also appeals to a man’s chauvinism as in his distrust of those not like him and to hold his culture as the highest culture. Islam teaches that it is godly to force others to accept Islam or kill them.
Islam also holds as acceptable to god that a man can take as slaves those who are not followers of Islam and to lie, cheat and steal from those who are not followers of Islam.
Islam also makes godly mans war like nature. Islam holds that it is a direct path to paradise to die in the name of Allah.
Islam also appeals to the inherent bias in a man’s nature that he is superior to the female sex. It is a part of Islamic law that the testimony of a woman is only worth half that of a man.
Most other religions have had as their primary goal to tame man’s nature and mellow man’s vices to make it possible for men to live peacefully together so that they may prosper. Not so with Islam.
Islam was started by a war lord and it canons show the marks of that warrior's character.
Or, more appropriately, in this case, lack thereof...
the infowarrior