Demographics matter in the long run, because of the obvious that a culture won’t survive if it has no members. Of course, it is not the only way to win; you can convert others, either by persuasion or at sword/gun point, think of all those infertile Jesuits. Highly successful memes use all three : as well as conversion by persuasion and by violence, both ancient Christianity and 19C secular modernity probably had better demographics not through higher birth rates but higher survival rates (in the former case, Christians probably cared for each other a bit more on average than Pagans).
Conversion at gunpoint doesn’t really do very well any more, as neocons found in Iraq; modernity (in liberal capitalist guise) has lousy demographics overall, but that hardly matters if it still converts, except locally. The Spenglerite/Eurabian meme is that the Islamic world now resists conversion to modernity, has a demographic advantage, and will conquer Europe through invasion (even if originally invited in…) and demographic advantage.
To totally resist the modern world, you need to separate pretty drastically from it, even if there is more than a touch of hypocrisy involved (had not heard that the Amish are keen on 16C medicine). True of some Christian sects and ultra-orthodox Jews, but not true, I would submit, to the Islamic revival mainstream or Christian evangelicals. As a result, they convert, but not clear that there is much if at all of demographic advantage (more data required, but note rapidly falling ME birthrates).
The Mormons are an interesting case of a half way house. Utah has the highest birthrate in the USA, but not hugely so (TFR 2.35), on the other hand one suspects that that all those clean cut missionaries going round the world’s cities have a dismal success rate. Given that their prophet arose in the well documented and (semi) rational 19C, even those who believe in 1C dying-and-reborn gods and 7C warlords as Prophets, would regard Mormon beliefs as pretty fruitcake. So why don’t all those Mormon kids defect to the mainstream?
- most people are not rational
- what matters is whether they live in a successfully functioning culture
The downside to being a Mormon is that I do not see how you could be a proper objective scientist with those beliefs, but that affects few people. Otherwise it is oddball but fairly mainstream American Puritan – can’t have a cup of coffee, but not a word against usury.