1) Most of the decline is due to the death of older adherents to mainline Christianity, which attracts few converts because it avoids distinction or conflict. I'm talking about bland Catholic parishes and dioceses as well as bland Episcopalian, Lutheran, Presbyterian, Methodist, etc. congregations. For instance, how many young converts do the Congregationalists make?
2) Many of the same young people now showing up among the "unaffiliated" who wandered away from mainline blah-ness will show up in later as "Christian" as they get jobs and think about getting married.
3) The harder-core elements in Catholicism and Protestantism are growing, not shrinking. Their foot-print will continue to grow. In current American culture, they have enormous social and political vitality, and punch way above their weight as it is.
4) Seventy percent is a big majority. It's not a "working majority," but it's a huge one.
I think you’re right on all of that. What’s happening is that it has become more socially acceptable to walk away from religion, there’s no longer a social stigma associated with being areligious, athiest, etc.
So people who never really had much faith to begin with are walking away. Going to church is no longer required to keep up appearances.