You're right that the Episcopal Church, like all traditional mainline Protestant churches, has lost members. Back in the day, the Episcopal Church was called the (Rockefeller) Republican Party at prayer. If you wanted to succeed in business, the best place to start was a tony Episcopal Church, preferably in a tony suburb. Why else do you think Prescott Bush, Poppy's daddy, moved from Ohio to Greenwich, Ct. and promptly joined Christ Church, THE Episcopal Church for ambitious up-and-comers like Prescott? Back in Ohio the Bushes were Presbyterian, which was the thing in Ohio. When Poppy moved his family to West Texas in the seventies they all became Methodist because the First Methodist Church in Midland was THE place for ambitious politicians.

Nowadays you don't need to belong to the right sort of church in order to succeed in business, so once the sixties came and went the Episcopalians lost a lot of dead weight (though admittedly a lot of money) when the barely spiritual left