Why is it sad? Mainline Protestantism's erosion is entirely its own doing.
Have to agree for the most part. The Mainline churches have made a lot of very poor decisions over the past half century, even down to relatively mundane administrative stuff.
Mainline Protestantism's woes are definitely at least partly self-inflicted but I wholly disagree with describing them as "entirely" so. My theory of the mainline decline hinges on the Great Sort; mainline Protestantism is too "establishment" to become a Blue Tribe religion (like, say, Reform or Reconstructionist Judaism) and has staked out sociopolitical Stances too liberal to become a Red Tribe religion (like, say, the Southern Baptist Convention), so there's nowhere for it to go or to be except the "disappearing center" along with Conservative and Modern Orthodox Judaism, Pauline Mass normie Catholicism, etc. I tend to think pretty well of all those "disappearing center" religions because I think the sociological point of religion in America should be social cohesion rather than sectionalist enmity-mongering, but I don't think my reluctance to completely blame them for their own troubles is entirely due to personal fondness.
Similar, but related, I would say that the cause of Mainline Protestantism’s decline is that as, over the past 50 years, more and more Americans have become irreligious, it has been from these denominations they have mainly been drawn. As a population becomes less religious, those left are likely to be more extreme in their beliefs.
I would argue it's not about simply extremity of beliefs, but that Mainline Protestantism doesn't fulfill a core emotional need in the same way other branches of Christianity do.
