You know people were saying the same thing about Catholics in the early 20th century, when they were having 5 and 6 kids at a time when Protestants were having 2 and 3 kids. Of course, Catholics stopped having 5 and 6 kids, just as second-generation immigrants have family sizes the same as their native peers (if only because it's too expensive to have big families). 

The Catholic takeover of Canada (and the U.S.) was a bogeyman that terrified nativists. It proved to be unfounded alarmism. So will the Muslim takeover of Canada.