I wish I could say this would be the perfect opportunity for progressive Christians to take over the religious scene, but unfortunately it likely won't be that easy even with this new information at hand. We seem to have entered a realm in which self-described liberals feel they have no need for religion, and those who do have faith are heavily concentrated in the very conservative denominations.
This opening has been recognized and is being taken advantage of: http://en.wikipedia.org/wiki/Emerging_church
BRTD, from what little of I've heard of the emerging church, they seem to be doing a good job of bringing in new converts. How are they in the getting people married and having kids side of things?Not as bad as you'd think, but not as good as one would want for healthy growth, I'll give you that. However for new movements that's not as big of a deal since they can pick up growth from elsewhere. You mentioned disgruntled evangelicals in your post. Well where are many of them going?
Let's look at the Catholic Church's problem. Though the stereotypical large Catholic family is a thing of the past, its members still probably have a birth rate at least higher than average. It loses 1/3 of all its members raised in it, but in the US that's not particularly high nor a problem. Plenty of evangelical churches have similar numbers and some mainline ones are even worse. The difference is that both of them pick up a significant number of converts, which in the case of evangelicals more than makes up the difference, and for mainline churches at least stems the bleeding, which would be FAR worse if they had a conversion rate like the Catholic Church's. In other words conversion is perhaps more important than birth rate, since you can't guarantee all of those kids are going to stick around.
To use a crude and stereotypical of me, yet actually fitting in this example, look at my whole "scene". The birth rate is probably significantly lower than it is amongst people in that age range nationwide, yes. Not very many people are "born into" the scene. Did it die out in the mid-80s? No, because it keeps getting "converts" and keeps growing based on that.
To use another crude analogy even though I'd rather not make this comparison: Does Scientology rely on high birth rates of its members for its growth?