I agree with most of the sociological analysis; but there can be a good discussion on the connection between that and the theological situation.
Until the 1960s, almost everyone "went to church" or had some connection with the church and that was part of "life." (They also joined bowling leagues, the VFW and American Legion, PTA and fraternal lodges, also a part of "life.") There was community disapproval leveled at those who did not do those things.
That is no longer the case.
Some might contend that the social and intellectual upheavals of the 1960s contributed to the decline of the church, as standard beliefs were questioned and challenged. But I contend that many found the church's engagement with the world appealing, refreshing and stimulating; and that the new approach to theology - now seen as discussion, give-and-take, and an intellectual exercise rather than unquestioning "faith" - kept many people connected with the church, which they now saw as more relevant and "modern" (not a bad word).
It did not all go well. As with the Reformation, some took the "new" ways too far; others used them as an excuse for abandoning the church altogether.
Others continued as before and others who had bad experiences in the church (see the new book "unChristian") could leave without societal disapproval.
The social and intellectual turmoil also stimulated a reaction; and a new kind of "conservatism." Many found the questioning a great threat, and sought places where they believed they would get clear, black-and-white, yes-or-no answers to worrisome problems.
As to whether the decline in mainline church is "massive," and if so, the meaning of that decline; I don't think any of our analyses are compelling. Is there a "new Reformation," and if so, is it the rise of evangelicals or the rise of socially and theologically tumultuous church bodies? Who is playing the roles of Luther, Calvin, Zwingli and those folks; and who is attempting to preserve the "church" against the upstarts?
Personally, I'm sure not willing to "let any sheep die." Some may find God calling them to another kind of expression of faith than now exists in many "mainline" churches. Sorry to lose them, but they are still part of the body of Christ. Others may be finding what might be considered "new" expressions of the Christian faith, with elements (like ordination for women, gays and lesbians) that trouble other parts of the Church. They, too, are still part of the body of Christ.