https://timesandseasons.org/index.php/2021/08/is-the-church-replacing-itself-in-the-united-states-population-momentum-and-its-capacity-to-hide-decline/

"As I’ve noted elsewhere,[2] raw on-the-rolls membership is not the best indicator of Church growth for a variety of reasons, but increases in units is probably a good indicator of substantive growth, and by these measures the Church appears to be doing fine.

However, this superficial reading can be misleading, as it does not take into account “age structure effects” which may hide very real declines in Church growth that are not readily apparent from a surface reading of the numbers. Age structure effects is a very technical subject, but to summarize: the Church in the US may be forming new wards, registering more members on its rolls, and building more temples to meet increasing demand while actually having undergone changes, such as lower birth rates and a net outflow of members, that will lead to the US Church shrinking once the older generations die off."

"If the pyramid “base” is conceptualized as an increase in “children of record,” that is reported in every general conference, then that suggests that the Church as a whole is pyramid C, since the “children of record” numbers have been relatively flat for the past decade. If this is the case in the US, then most if not all of the growth that is fueling the expansion of wards, stakes, and temples in the US is essentially being caused by childbearing that happened in the past, and once these effects pass through the system growth in church units will stagnate, potentially to nothing, and we’ll enter a time period where no new branches, wards, or stakes are being added"

"Specifically, according to the UN projections, if the United States fertility and migration rates had immediately changed to replacement level on January 1st, 2019, the US population would grow .5% a year purely due to population momentum.[5] Consequently, the Latter-day Saint community in the United States would also grow at least .5% purely because of population momentum.

Comparing this to the reported Latter-day Saint growth sheds some light on the actual rate of Church growth. Specifically, in 2018 the US Church grew about .6% in terms of the absolute number of members on Church records, and .3% in terms of additional branches and wards.[6]

In other words, it appears that the currently observed level of growth in the US Church is approximately equal to that which the Church would expect to experience from population momentum alone; if the growth in units is assumed to be a more accurate indicator of growth in active, practicing numbers then the Church appears to be actually contracting in the United States, with the population momentum effects hiding the decline. It is worth noting that, as it is likely that the Latter-day Saint population is younger on average due to members’ well-known penchant for larger families, equating Latter-day Saint momentum with the general US population’s momentum is a conservative assumption: it is probably higher and therefore the proportion of Church growth that comes from momentum effects is probably as well."

"Second, while the Church will probably continue to enjoy strong growth in the developing world for the foreseeable future, the fact is that the Church has been financially blessed to have its population base in one of the few very wealthy countries with a sizable religious population. As momentum effects wear off and if the Church enters a decline in the US, the tithing spigots will turn off. It is for this (and other reasons) that I am less consternated about the Church’s reserve fund than others. Yes, $100 billion seems a bit extravagant for a “rainy day” fund a la the 2008 crash. However, there is a very real chance the Church will undergo gradually increasing, very real financial pressures as the relative number of lucrative US tithing payers who have traditionally subsidized the Church’s operations in developing countries declines."