There's a big difference between church attendance and church membership... Unfortunately, the original linked article glosses over the difference, while the original Gallup study digs into it. According to Gallup, approximately half of the recent reduction in church membership can be attributed to people becoming less religious, while the other half is explained by people who still attend church regularly, but avoid formal "membership" in a congregation.
And, to your point, the historical data is very telling. My understanding is that the idea that basically everybody in the US used to be super religious and go to church all the time is largely a myth. In fact, I believe the high-water mark for US church attendance as a percentage of the population was actually around 1990. (Source: Sociologists of religion Rodney Stark / Roger Finke. Their book "The Churching of America" attempts to get historical stats on this question, and includes this graph: https://madeinamericathebook.files.wordpress.com/2010/03/fin...)
Take from these stats/trends what you will. Just adding some additional context to the discussion.