Well... Maybe. You have to keep in mind, at least in my experience, people who answer "none" still often have strong religious beliefs. They don't go to church/temple or identity as religious or know the rules or ceremonies but hold beliefs of whatever aspects of religion that personally appeal to them. My dad is one of them. When asked his religion he'll say "none" but he believes in God, spirits, and prays.
So this may be just a gradual move from organized religion to unorganized religion rather than a move from religion to non-religion.
(Hell, even the people who are part of an organized religion do that sometimes. I have a Catholic friend who believes in reincarnation. I guess that's how you get new religions.)