As I see it, the numbers of people who those of us in the church would say are actually committed Christians—those who are practicing a vibrant faith—are not dying off. The Church is not dying. It is just being more clearly defined.
The "Nones" category is growing quickly, but the change is coming by way of Cultural and Congregational Christians who no longer feel the societal pressure to be "Christian." They feel comfortable freeing themselves from a label that was not true of them in the first place. Convictional Christians are not leaving the faith; the "squishy middle," as I like to call it, is simply being flattened.