Earlier quoted context omitted.
I think the issue is much more what will happen to the world's numbers before the population numbers decline (IF indeed EVERY ethnic groups starts reducing numbers. God help us all if there is one that doesn't) Numbers of fanatically religious people will double, relatively speaking. Mostly Catholics and Muslims. This is under the assumption that the current rates of conversions remain roughly the same. Trouble is, p…
It's interesting, the generalized idea you have about "Catholics" and "Muslims" comes from a point of view where they're a minority. I'm from Mexico where almost 90% of the population is Catholic and something interesting happens here: Protestants are the minority and they're viewed as the "fanatically religious people", people would say they're always talking about their religion, they don't party a lot, they don't…
It probably doesn't need to be interpreted as "radical" or even "fanatic", it actually is radical and fanatic.
I grew up in a country of 90+% Catholics, but I was raised in a minority Christian religion. Yes, we always talked about the religion, didn't party (that) much, etc. - that was because we actually cared about following our beliefs, which can't be said about Catholics in my country.
Thinking more about it led me to the following conclusion: any religion that grows so big that it becomes mainstream has to relax a lot. Religious life becomes intertwined with day-to-day problems of employment, managing the economy, etc. Meanwhile you stop having small communities that can push all unconvenient issues to outside with "us vs. them" attitude. Professional specialization starts, people are delegating their religious issues to their priests (compare the way Catholicism looks in big cities vs. small villages). And then one day you wake up and can't ignore politics anymore, because suddenly your religion is the society, and thus is the politics.
TL;DR: Going from "more fanatic / more real" to "less fanatic / less real" seems to me to be a natural course of evolution of a religion as it grows.