Church attendance and church membership -- especially in historical churches -- has been in steady decline for several decades, more or less coinciding with the dawn of the status-striving era, beginning with the Me Generation of the 1970s.
This shows that their decline is not a response to very recent trends where the church tries to play catch-up with the increasingly degenerate mainstream culture. Rather, the churches were abandoned by strivers who up-ended the idea that members would fit into the church, and insisted that the church fit them. This change was just one of a whole pattern of reversals where individuals insisted on fulfilling their own ambitions rather than make any sacrifice toward accommodating the group.
After awhile, the churches got the message loud and clear, and decided to chase after the me-first population that was becoming more in favor of divorce, abortion, sodomy, homosexuality, gay marriage, and so on.
It's important to emphasize the relative timing of these trends for younger people, who do not remember there ever being real churches. They assume that the Mainline churches were at the forefront of multiculturalism, fat acceptance, and tranny rights, and that's why their members have been leaving in droves for decades.
In reality, the Mainline churches only recently adopted progressivist stances, and some still have not -- gay marriage is still forbidden and homosexuality considered incompatible with Christian teaching, within the largest Mainline church, the United Methodists. Yet their membership began declining sharply during the '70s -- long before any church was pushing for gay rights, let alone the UMC which still does not sanction homo marriage.
Aside from the general observation that the status-striving climate has led to the decline of traditional churches, what can be said about the different groups or waves of strivers? Recall the career vs. lifestyle strivers, and now the persona strivers.
The first wave of strivers fought their status contests within the arena of career, wealth, and conspicuous consumption. They were mostly Boomers. Once they left the historical churches, they could only be won back with a prosperity gospel -- one that promised that their attendance at a newfangled non-denominational service would be rewarded by God with career and material success.
These services increasingly took place in mega-churches, beginning in the 1980s, with the preacher playing the role of a motivational speaker to an audience looking to get rich quick. The size and spectacle of the mega-church is meant to signal the material success of the founder, to convince the audience that when it comes to the path to prosperity, he knows whereof he speaks.
The next wave of strivers waged status war regarding lifestyles, fashion points, and conspicuous leisure. They were mostly Gen X-ers. Some of them didn't actually leave the historical churches, but transformed the way that services were conducted. They weren't seeking a prosperity gospel since they had no delusions about being able to get rich quick. Their focus on novel lifestyles and prioritizing leisure led them to target the rituals and form of worship itself. This led to the shift toward "contemporary" worship services, as opposed to what are now called "traditional" services.
Contemporary services may take place in an old Mainline building (or a newer, mega-church-ier one), but they all eschew participation by the audience in public rituals. Instead, the audience remain passive spectators of a performance that is part motivational speech (not geared toward prosperity, but how to live a better life), part stand-up comedy, and mostly a concert blending pop, rock, and country music genres. The songs are all new, and they are only generically spiritual -- lyrics about God, Heaven, etc., but nothing distinctly Christian about Jesus' life, teachings, and resurrection. It's more fashionable music that meshes better with "belief in a higher spiritual power" lifestyles. This shift began during the 1990s.
The recent wave of strivers do not pursue career success or the most au courant lifestyles, but rather a quest to craft the most unique and awesome persona, and to preen before a panel of peer jduges -- these days on social media, but during the fin-de-siecle by hanging out in salons. They are mostly Millennials. They do not generally belong to any church; those who do, tend to treat it as a cosplay event at a religious kind of comic-con. Rituals, practices, and services mean nothing to them since those are all elements of a regular lifestyle. A prosperity gospel would fall on deaf ears.
If they aren't participating in corporate worship, nor even listening to a guru preach, it is because these things add nothing to the individual's persona. The focus is instead on studying a range of elements that could serve as a patch within a larger garment that the wearer can show off. They do not even need to come from the same religious tradition -- maybe they'll try to maximize their special snowflake points by cultivating an image of Buddhist-Christian fusion. Or some obscure group like the Gnostics, Manicheans, Zoroastrians, Norse cults, Druids, or whoever else would allow you to make a costume that would drive the other dresser-uppers jealous. This shift began during the 21st century.
These three separate waves of striver-driven corrosion would be best studied within a church that still draws large numbers of people, so that all three waves could be seen over time. The Catholic Church would serve well as a case study, although I'm not very familiar with it.
The prosperity gospel, mega-churches, etc., has been primarily a Protestant phenomenon. But there had to have been something akin to it in the Catholic Church.
Certainly there was a shift toward "contemporary" worship styles that took over the Protestant churches during the '90s -- meet and greet, more familiar music genres (if not an outright rock concert), and sermons that are less sacred and more informal, mixing stand-up comedy and motivational lifestyle speeches.
And a good fraction of those who make religion part of their persona striving, have chosen to adopt a Catholic identity -- usually an obscure one, almost always pre-Vatican II. Plenty of born-and-raised Catholics long for the old days, too, but it's not part of a persona-shaping project. The pre-Vatican II yearning converts (or maybe not-yet-converts) seem more like they're into Vintage Catholicism as a persona marker ("trad Cath"). Nutjobs like those who believe there is no legitimate Pope today (sedevacantists) are unlikely to take regular part in the rituals, practices, and other lifestyle elements of being a Catholic. It's pure persona creation -- "I'm the kind of person who feels like..."
Ultimately, the churches will only be restored when people reverse the status-striving trend and begin to think about how the individual can fit into a group rather than how the group can change to recruit the individual. The Catholic Church's traditions are so old and widespread that they could recover them in a new climate of accommodation and sacrifice. Some of the Mainline churches have hymnals with traditional music and lyrics that could be dusted off after the rock concert loses its lifestyle striver appeal.
I'm afraid, though, that anyone who left a historical church altogether (rather than "merely" changing it from within) is going to have a hard time going back. And the novel mega-churches that they spawned will probably collapse when the "what's in it for me and my success?" approach to religion fades away.
It's striking how absent the Boomers are in historical churches these days. The Silents and Greatest Gen are in the "traditional" services of a Mainline church, while the Gen X-ers are next door listening to a guitar-and-vocals concert. The Millennials are absent, too, but they weren't very present to begin with.
I don't have a strong hunch about which self-absorbed generation will find it harder to return to normal churches. Boomers don't want any demands on their lives that would cut into career and material striving time. Millennials don't want to join a group and be prevented from attention-whoring and sharing selfies.
But probably the Millennials will find it easier to join a normal church simply because they aren't as advanced in age, and have plenty of time to have an epiphany about how pointless their persona diddling will be. And having grown up deprived of all social contact due to helicopter parenting, they must be desperate for normal group interactions.