Regarding the Islamic world. I doubt the ex-Muslim movement will ever reach the numbers required to challenge the majority of Muslims. But if they do, they would probably have to do it in alliance with moderate Muslims who want to get rid of the Islamist or oppressive rulers.
Not now, not in a couple of centuries, but sooner or later Islam will start to devour itself. The only Muslim who is worthy of any respect at all is one who has the guts to reject the worst aspects of Islam. Conformist sheep who do not strive to enforce Shariah, but who do not really oppose it either, are not good allies. Contrary to what you think Hassan, moderate Muslims do not have any drive or motive to overthrow Islamic tyranny.
At any rate, you don't need massive numbers to launch a violent offensive against Islamic sheep and shepherds --you need dedicated, angry radicals with a good range of explosive equipment. I am not talking about civil wars, I am talking about asymmetric warfare.
All the successful models of secularism in the Islamic landscape have persecuted ordinary Muslims to some extent. It is high time Muslims get a dose of their own medicine. Time will come that a Muslim trying to rape "exposed meat" will be shot on the spot, a mob of Muslims trying to lynch apostates and infidels will be gunned down in return, and people who spout conspiracy theories and xenophobic hatred will be ridiculed and humiliated everywhere they go. Mosques will be shut down, Islamic shrines will be desecrated, and hateful Imams will be dragged on the streets. Copies of the Quran will be appropriated, and publicly burnt.
Unlikely? For the time being, yes. But remember that similar events were commonplace during the French Revolution. Considering the culture of violence and retribution which dominates much of the Islamic landscape, a peaceful transition is quite unlikely. Islam will not die easily, but it will die.