Interesting.
I think the disappearance of Catholicism will probably take a couple of centuries. What you’re seeing now is the disappearance of
anglophone Catholics in the U.S. and Canada, which will probably be complete in about 50 years. That’s not really comparable.
religions.pewforum.org/portraits
If you look at the data here you can draw some really interesting conclusions. Catholicism is disappearing three times as fast as the next highest denomination. (10% of Catholics have left Catholicism, just in the last generation).
Catholics seem to have a slightly higher birthrate than the mean, but they’re also quite a bit less educated and wealthy too.
Bear in mind that there are French and Spanish speaking Catholics in North America who look upon religion as an inherent aspect of their ethnic identities.
As the children of immigrants grow smarter and richer (to put it bluntly) they might end up leaving too.
European Catholicism will probably be allowed to survive as a small, minority religion even under an Islamic Europe (which is projected to happen in 50 years).
weeklystandard.com/Content/Public/Articles/000/000/004/685ozxcq.asp
So you’ll see the Vatican paying the jizyah tax for a century, before finally fading away. Or, perhaps it will hang on, like the Christians in the Arab world do, as a tiny minority which segregates itself along religious lines. Muslims are far more tolerant of Christians than vice-versa.