My 2 cents on some of the decline in church attendance and decline of Boomer Christianity (evangelical).
1. Excessive altar calls. In the early 2000s I attended Jerry Falwell's Liberty University. We had to go to church service every Sunday and 3 chapel services on Mon, Wed, Fri. At the end of every service, all year long, every year, the pastor conducted an altar call with accompanying sappy music. He often used phrases like "do you know that you know if you died tonight you'd be in heaven". I heard of people going up a few times, because, hey, were you really sincere that last time you said the Jesus Prayer? Well, I've been living like a Christian, trying to follow His commandments, etc. Ah! Ah! Ah! That's works bro, that's works.
It didn't take long for me to sour on the Jesus Saves message. I suspect it didn't take long for others, either. Then it dawns on you, why am I listening to all of these sermons if all that matters is the altar call? If I've altar called a few times, and I'm saved, what's the purpose of church? I guess to keep the altar call cycle going. But then let's do the altar call, a quick baptism station and be done with it. Can you spare a minute to get saved? We'll give you a free towel with the Star of David on it. The altar call cheapened church to the point where it's superfluous (the church). If Christians aren't attending church it's because there is, quite reasonably, no reason to do so. You believe, you're saved by grace, now go on and have a blessed day and don't come back now unless you're ready to tithe.
2. Debating atheists. A lot of boomer Christians got caught up in the (((atheism))) debates. By trying so hard to get people to believe God exists, Christianity and living a life for Christ is completely overshadowed. Now the debate is on the age of the earth, the efficacy of science, philosophical problems like the problem of evil or suffering, and examining the historical-critical interpretation of the Bible, the historical method, historical evidence, semantic debates, and on and on. I got caught up in this and felt, "well, what's the point in the rest of it if you don't even believe God exists?" But now we know what the point of the rest of was because of how foul and corrupted our culture has become with the woke and Clown World stuff going on.
I think the 'once saved, always saved' stuff has failed. It didn't conserve our society and culture or even Christianity and so American Protestantism begins to decline. Focusing on whether or not God exists moved the goalposts so far away people even lost power over pronouns. The culture has become clearly demonic, meaning there must also be an opposite, which isn't just what's traditional, but what's Christian. And yet that seems lost, which is why I'm an orthodox inquirer, because I had never heard of theosis (and apparently neither has (((spellcheck))) ) before, I figure there has to be something beyond the simple salvation prayer stuff, which is a recent invention anyway, and that one has to do some kind of work or put in effort into their faith.
I will close with a small anecdote proving this point where some Christian guy produced his own movies. One of them was a movie where he, known as Gramps, goes to college. This was 2000s era. The premise of the movie is how this good Christian man is stunned by the sinfulness of the students and the college. It should be really good with a lot of great points being made all over the place, the field is ripe for threshing, or so I thought. The film quickly degenerated into where gramps debates a science professor who is teaching evolution. The whole thing becomes about this debate and how science is undermining Christianity. I think a lot of good people were duped into having that debate and it helped maintain a lukewarm Christian church.