@AmmonLam My gut still feels like it should be less than 50%, but I would want to do some actual analysis before betting more than a trivial amount on it.
Some things that I am uncertain amount:
(1) Changes to how the missionary program is structured have been more important in determining the number of missionaries & baptisms than long term trends, at least since 2000. Raising the Bar (2002), Preach My Gospel (2004), the Age Change (2012), and COVID (2020) are all visible, noting that many policies take 2 years for the effects to be seen. The long term trend is hard to see as a result of these changes.
(2) The Church feels like it is doing less now than it was in the 1960s/70s to encourage its members to have children. For example, birth control/contraception hasn't been mentioned in General Conference since 1994. If the Church does find itself with a below replacement birth rate, there might be some actions that the Church could take to raise it.
(3) Official membership is growing more rapidly in Latin America & Africa than in the US. Is this real growth or is retention so bad that it won't compound on itself? If a population is actually composed of two subpopulations and the larger one has a smaller or negative growth rate, then the overall population will look like its growing with the slower growth rate until the two subpopulations are about the same size, after which point, it will grow with the faster growth rate. (The growth rates don't have to be constant, but the principle still holds.) This has already happened with the Catholic Church, which transformed from a majority European to a plurality Latin American Church over the course of the twentieth century, and now Africa is becoming increasingly important.