I think the drop in apparent numbers is due to the fact that not belonging to any church is much more socially acceptable than it was just a few years ago.
Used to be that folks were ashamed to say they weren't believers, so even if they hadn't been near any church since they were baptized or went to Sunday School as a kid, they would fudge and claim to be some variety of Christian.
Now they're emboldened by the social acceptance of atheists and just lukewarm nonbelievers. They'll still think up some excuse why they don't go, if asked, but they just don't care any more.
I think the drop in apparent numbers is due to the fact that not belonging to any church is much more socially acceptable than it was just a few years ago.
Once, people would have thought themselves Christian, when they were in fact lazily irreligious.
We've been through times like this before.