I think it's clear the world is becoming more secular. What's also true is that the void is being filled with other things. People don't just go from Christian to atheism/agnosticism. They often go from Christian to unorganized variations of paganism, or pantheism, or some hybrid of various spiritualities. The problem with these from a societal standpoint is that people will often adopt one of these because they "feel" right in the moment. What they lack, however, is a comprehensive body of theology or philosophy which helps to form the basis and nuances of the belief.
I think Christianity helps to fill an innate need for us to know that we belong within the bigger picture. Without Christianity, this need is still there. People who leave Christianity still need to fill that need, and they will always be searching until they rediscover where it was all along.