The 'Spongian' elements of progressive Christianity have certainly been no help in winning over converts and keeping people and their families in the pews. I am of the belief that a church can be 'open and affirming' to LGBT people while maintaining a theologically orthodox message that offers a real and enticing alternative to secularism. In that regard, mainline churches have failed.
The Church of England has actually managed to attract young people by going back to basics and rejecting modernist theology that denies the divinity of Christ and the Resurrection. Because absent those things, there is little incentive to join a church that merely reinforces modernist norms rather than preach the Word. Mainline churches ought to take lessons from the CoE and Billy Graham if they want to stop the bleeding of membership and believers.
There’s also the fact that secular people and religious liberal people have far fewer children than, say, evangelicals do. A facinating book on the subject is called “The Religious Shall Inherit the Earth.”