I'm not following you. The older generation will have to give way to the younger generation eventually. At some point, we're going to be the arbiter of values; I don't see how that's escapable.
If you look at Kenya, there are more people under 30 then everyone else combined, the 4 age group tires between 20 and 34 form the majority of adults, as it used to be in most of the western world. Under that pyramid structure with the base - the youngest - the biggest tire of the population, which meant that upon reaching adulthood it took a very short while for them to become the majority of adults, enabling them to critically dominant and define the prevailing culture at the time.
In the skyscraper structure, the same generation, people between 20 and 34, are not really a significantly larger population then the people between 35 and 50. Instead of a new overwhelming majority, they are minorities sitting side by side. And you are correct that they will get older and if you continue to add each tire of "new adults" into their numbers, they will eventually outnumber those above them - that is the line gets pushed up and when they become 49, if you count everyone between 20 to 49 they would indeed be the new majority of adults.
But if you want to account for shift in culture & values, is that the correct way to count them? After all, in the pyramid structure the older 3 tires that gave way to the new majority were once the new majority, which you could notice if you kept looking at the same 3 tires over the years. Likewise, as time progresses the newer generation matures, and just as millennials differ from those older so could gen z differ from millennials. So instead, when the people between 20 and 34 become 35-49, they have a new generation with their own cultural shift and values between 20 and 34 on one side, and the older generation - slightly smaller if at all - between 50 to 64. Neither one got to be the overwhelming new adult majority, they are instead multiple minorities living side by side. Even if each age block is drastically different from the next, neither one of those ever gets to become the overwhelming influential majority of consumers, employees, voters defining our culture. They are each a minority voters, each a niche market. To apply this to the topic at hand: In order for the non religious values among millennials to ever become the majority, we'd need the next 3 generations to feel exactly the same.. And culture is rarely that linear in it's paradigm and value shifts.
More so, the better we get at extending life, less influence each block would have, a 15 years generational block was a lot more influential when the average lifespan ended at 30 then when it ended at 75, each of those blocks is becoming a smaller less influential minority. And all this post you might be asking... Why the fuck do I keep using a 3 x 5 years block? Generations measured by 25 or 20 years, sure... But 15? That's never used... Which is true and brings me to the next point:
If we continue to face accelerating changes in the ways that directly impact the culture of the kids growing up within it, if we are getting faster and faster at generating new ideas, content, art, lifestyle options, means to communicate, dreams, ambitions, and fears, and all the things that inspire and enable new values to grow on within the new generation... Then the block size in which we can make any cohesive generalizations of culture and values should actually become smaller. For a more extreme example for simplicity's sake, If the people ages 20-to-40 have faced twice as many cultural impacting changes as the people 40 to 60, and the breakdown of exposure and influence by those changes is more or less divided by age, then the people in their 20s and can be as different from the people in their 30s as those are from people in their 50s. It makes more sense to measure generational cultures by increasingly smaller time intervals, which means that not only can the defining characteristics of "the new generation" no longer be the overwhelming majority of society, it is now morphing into a smaller minority. This is what I mean by new individuals changing faster but society as a whole changing slower.
(Unless ofcourse the new kids decide their new thing is to go back to families with 10 kids at which point they might restore the pyramid structure and the whole point is mute... But unless something happens like immortality or full body and mind cloning/replication or space colonization or a decree to kill everyone older then you becomes the new religion... The gen to restore the pyramid is probably not going to be millennials).
