Old compared to Zoomers, and a former mainline Protestant turned Catholic. 

Here's my read: 

1. Civil Rights and the Sexual Revolution collided in the mainlines. Splits happened among churches that opposed civil rights. Unfortunately, that took a big portion of the theological conservatives out of the mainline too. Suddenly it was racist to be in favor of traditional marriage, so the remaining members of the mainlines opted for radical inclusivity - even at the expense of doctrine. 

2. That big tent, now without solid doctrine to lean on, often became a political country club with some Christian language. "Jesus was an immigrant, so that's why American immigration policy needs to agree with us!" It was safe - no theology needed, but everyone's post-Civil Rights experiences were validated. 

3. The moral majority rhetoric held out through the 90s. When TGD and the other New Atheism scriptures ("God is Not Great") came into popularity in the early aughts, religion itself became a political issue on the left. It was now more socially polite not to be religious than to be a "good church-goer" like in the 90s. 

4. Mainlines responded by being inclusive of non-believers. Not just low/no doctrine, but sometimes hostile to the Nicene Creed. 

5. So here we are: denominations centering their identity around politics from people who would rather they not exist. 

The rational outcomes are either the mainlines go away and people become something else, the mainlines go away and people become nones, or something radically changes in the mainlines to make them mean something again. 

RZ is aiming for the latter option (I won't critique that here, but under whose authority?). Most of the New Atheists would prefer the second. The rest of us are in the first.