Actually, this is already happening. Christians are increasingly leaving congregational sized churches to move into either larger or smaller settings. This is occurring all over the country. Mega churches are doing quite well. Those who attend these churches appear to desire large gatherings, loud music, generally gifted orators, and whatever the latest trends are. These churches offer a wide variety of services and programs to various age groupings. I don't hear of mega churches struggling at all. Rather, they appear to be growing as people consolidate where they are gathering.
I also hear about increasing numbers of believers leaving institutional churches in favor of much smaller church bodies. These folks usually crave simplicity, family settings, edifying gatherings, and authenticity. While these types of church bodies tend to vary quite a bit from one to the other, the commonality of simplicity remains. Christians are increasingly finding out about this type of church life and are embracing it.
I'm a church growth enthusiast only if it refers to spiritual growth. However, as we interact with both the church and the world, it benefits us to be aware of current trends. We Christians have a disturbing tendency to operate way behind the curve when it comes to almost everything. Let's stop making that mistake. As we move forward, it is simple churches and mega churches that will be most vibrant. We should be aware of this and seek to know why this is happening.
Why, then, are mega churches and simple churches growing? I believe the answer lies in their one commonality: community. Mega churches often offer a wide variety of small groups/life groups/home groups that function something like simple churches. These groups, if operating well, provide the family type of atmosphere missing in much of life today.
In the end Christians desire community in Christ. They are finding this in mega churches and simple churches. Therefore, these types of bodies are growing.