How will the kingdom of God continue to grow until Christ's return given the statistics at the beginning of Gaining By Losing about the predictions of churches closing their doors and fewer and fewer people attending church each weekend? How will His kingdom grow continually if most of Church growth is shuffling believers from church to church?
A couple of thoughts in response to your question. First, the Kingdom of God will continue to grow because God has promised that it will. God has promised that there are people of every tribe, nation, people group, and tongue still to become citizens of His Kingdom. So we believe the Kingdom of God will continue to grow because God has promised it will.
Second, and much more practically, the Kingdom will grow when more Christians start sharing their faith. On page 26, in Gaining by Losing, Greear mentions that "90 percent of evangelicals have never shared their faith with anyone outside of their family." Maybe the reason we're not seeing the Kingdom grow in our country is due to us not scattering the seed of God's Word.
Finally, the statistics on page 27, about fewer people attending church each weekend, is specifically about the United States -- not about churches all around the world. In the global south and east Christianity continues to grow as believers in those areas of the world boldly proclaim their faith and share it with others. So the Kingdom of God is growing. If we want to see growth once again in our country, we need to get back to doing our part of scattering the Good News and prepare ourselves to disciple those whom He saves.