Presume that entities develop through varying rebirth processes, and that, contra the remnants of Indo-Arya, humanity isn't the stage immediately preceding Nirvana, even though it's higher than dog or rat or rock. What do we see during any such process where individual units differ from one another, and where output is constantly expanding in accordance with the expansion of the universe? We see a pyramid shape; graphically, a reversed exponential function. At the bottom of the pyramid is vacuum, or empty space--space in which this verse's physical and temporal laws are laid out, but where energy and matter aren't active. A little above that, we see basic energy or material patterns--lots and lots of them, but not as numerous as mere empty space. Another few steps above that, we see simple elements, then more complex elements, et cetera, for as the universe grows--adding, say, a septillion cubic kilometers of empty space per second--empty space will always dominate, but by being created will provide venues for energy and matter to develop, so the simplest forms of the latter pair will secondarily predominate quantity-wise.
Then up the pyramid come more focused expressions of matter and energy--denser elements and more powerful energy reactions--and those are more numerous than, say, complex planets combining many dense elements, which are themselves more numerous than complex planets combining many dense elements which also include very simple forms of "life," which themselves are far more numerous than more complex forms of life, which is why there are more insects than poodles, more bacteria than humans, and more drums than harps. This perspective doesn't disparage drums, regular football viewers, rocks, or hydrogen, anymore than it exalts regular Wagner viewers, platypuses, or uranium, but rather identifies them on a scale of development through which all have or will have passed.
Within each divided gradient, the pattern would visually resemble a bell curve rather than a reversed exponential function. For example, among all hydrogen molecules in existence at Point A in time, the number whose nucleic protons are extremely "loose" would be at the left side of the graph, the number whose nucleic protons are extremely "tight" would be at the right side of the graph, and the number whose nucleic protons are pretty generally stable would predominate, sort of the way the bulky "middle class" of a "results" bell curve would develop in a one-week-long fair resource competition game where genetically similar individuals started from identical starting points.
Pursuant to the rebirth model of versal development, an entity which has mastered a given stage of existence, and is ready to develop into a more complex expression of energy, then exits the prior gradient and moves on. In Hinduism, this means moving up a caste, or becoming a chicken instead of a dung beetle, or a tiger instead of a fish, and stuff like that--all as part of the process of pursuing as-yet-unfathomable Nirvana. In our expanding universe, this means that each segment of the development of energy patterns will result in a point where development is limited, then cut off, as there are fewer, then no, spirits/souls that need to re-do the highest stages of a form they've already mastered. E.g., once you've accomplished the task of maintaining physical and temporal laws, it's time to try out being an electron, then an atom, and so forth, ergo once you've mastered being a dog, you don't keep coming back as a better and better dog. The category "dog" will, therefore, never exceed a certain complexity level, because dogs of such complexity have already graduated to dolphins, or chimps, or humans, or whatever.
This karmic model can help us to evaluate birth rates. Without believing in evolution, you can still draw amusement from the idea. The question here is, "Why do better educated and more capable people have fewer children"? This pattern occurs inside European and American countries, both before and after modern consumerism to some degree, and it also occurs globally, for example, comparing Ethiopians to the Japanese. Imagine, just for fun, that the answer is, "More intelligent people have fewer children because, beyond a certain point of development, there isn't much demand, if any, for new shells."
The biggest unpleasantry to contemplating this is the high correlation between low-birth-rate populations of cruel selfish idiots. Remember though, this development need not be positive. More complex negative development is as universal as more complex positive development, and just as the stupid & mean, yet fiendishly clever, academics and bankers have much fewer children than the stupid & mean, and functionally illiterate EBT users, the sweet & nice, yet brilliant, whoevers have much fewer children than the sweet & nice, yet functionally illiterate, whoevers. When someone reaches the transitional zone for "human," whether down the path of good or evil, they move on to the next stage, ergo given the reverse-exponential and bell-curve graph models we discussed earlier, there is a dearth of within-type advancement, ergo fewer births requiring complex cognitive machinery. If this silly religious model were true, we should always see birth rates drop when intelligence and existential contemplation rise, and we should also see mysterious incurable diseases administratively destroy the complexity (and resulting capabilities) of redundant biomass vessels. In short, higher IQ groups should not only have lower birth rates, they should also have higher occurrences of complex diseases with graphable hereditary risks, representing both the regulators' elimination of unnecessary shells, and their dumbing-down of high-functioning shells into usable development modules for newcomers.
The impossibility of blending this idea into Terran expansive religions arose from the arrogance of considering humanity the highest developmental step before paradise, merely because the assumption that all available environments are contained in Terra, and perceptible to humans. Granted other venues for development, the progressive vanishing of more evolved individuals and groups would be as unavoidable as it would be unlamentable.