One thing that might affect the trend is immigration from religious countries, but I'm not sure there's enough of that to make a difference. It's also not impossible to have a revival of interest in Christianity, as happened with "The Great Awakening" in the 1700s, and again with the "Second Great Awakening" in the 1800s. As an observer of trends, I've noticed that conservative Christianity (fundamentalism and non-denominational fundamentalism) seems to attract more people; but there is constant churn as they constantly splinter and lose members.
Yes, good points. Still, I think that organized religion needs a critical mass in a population, otherwise enough people will feel free to question it and to visibly normalize their disbelief.
We saw this happen with gay marriage, where enough people realized that there were no objective, identifiable reasons to deny this civil right apart from some verses in an old book. When enough people realize the same about the rest of the verses, and their lack of authority for every other issue, then it will all fall apart from one generation to the next.
This happened in Europe after WWII, when people were massively disillusioned by "holy wars" and divine justifications. We're seeing it in the US now, with Evangelicals and their politics of hatred, discrimination, and vengeance that is driving young people away in droves.
