A bibliographic note: While it is no secret that I am dealing with questions that have been discussed many times before, a friend pointed out to me one book on the topic that has been on my shelf for years but I'd never opened, and that I (having now read it) would urge anyone concerned about the questions I have raised to read. Larry Hoffman's The Art of Public Prayer tackled these issue brilliantly in 1988, and is still right on the mark today.
It is unquestionable that Orthodoxy is the only sub-community in the Jewish world whose numbers are growing. That said, it makes me crazy when Orthodox leaders point to this phenomenon as evidence of the inherent correctness of their theology. It is a lot like the Church asserting that the conversion of the Roman Empire to Christianity in the 4th century constituted proof that Jesus was the Savior.
Underlying this is a deeper misconception. It is easy to assume that the reasons people belong to a particular religious community are primarily theological. Jews believe certain things about how (or whether) God gave the Torah, or about how (or whether) Halakha (Jewish law) developed, and therefore join particular synagogues. But more often than not, the opposite is the case. Hoffman argues that faith in God is not the product of rational deductions about the nature of the universe. Faith is generated when we experience profound moments of emotional high and spiritual fulfillment in a religious context. Those experiences lead to an awareness of God's presence within moments of transcendence, and to a desire build up our connection with God. Put simply, spiritual experience creates faith, and the beliefs we embrace about God are a largely a result of that faith, not their cause.
This is especially true of those who become Orthodox. Of the many baalei teshuva (those who join Orthodox communities as adults) I have known, most are very strident in asserting their belief in Orthodox doctrine. But few of them became Orthodox as a result of this belief. In search of something, they tried out or just stumbled upon a shul or yeshiva which offered experiences or a community which they found deeply fulfilling. Wanting to be a part of it, they took on the set of doctrines that their teachers taught them were required to be full participants. Thus whether one is convinced that Torah Codes are real, that God revealed all of Halakha to Moses at Sinai, or that the Lubavitcher Rebbe is the Messiah, often depends on who first enabled those profound experiences.
All of this demagoguery is to make one simple point. People choose communities based mostly on where they find social support and spiritual fulfillment. Indeed, for young parents theology is well down the list of priorities. But what community they choose will have a profound impact on the beliefs and identity of their children, who will grow up shaped by their parents' choices. And a community's beliefs will also, over time, shape the religious vision of those who find inspiration and fulfillment there.
I am personally convinced that what kind of Torah Jews will teach to the world depends on the kind of Torah being learned in communities of committed Jews. If we want a progressive and socially conscious Torah to have a serious influence on the Jewish future, our most important work has to be replicating the major successes of Orthodoxy: shaping profound religious experiences that draw people in, and developing real communities of shared religious commitment and values that make them want, even need, to stay.