Specific personality traits aren't generally considered
predictably heritable these days; at least, not in the direct gene-to-trait way that this would suggest. There are any number of ways that your genetic code might affect subjective expressions of personality, but personality itself is a complex phenomenon that is at least partially a social construct (ie, scientifically you don't really have "a personality" so much as a bundle of materially unrelated traits and conditions that the humans around you use cultural frameworks to "interpret as a personality"). There are also some studies that show a correlation between genes and personality expression, especially across certain spectra. But we're talking about very small statistical correlations; present and real, but dwarfed in effect by other factors such as socialization, enculturation, and individual experience or psychosis. Teleological errors are common when laypeople attempt to discuss genetics, and this particular area is one in which "gene for" type studies have proved especially useless.
So the kind of genetic manipulation you are suggesting here probably wouldn't work. It would work on other animals, to a certain extent, but even in that case it would likely take a very long time - note for instance seven centuries of unsuccessful attempts to breed docility into American bison populations, and that in much smaller and less variable breeding communities than even the tiniest of human village networks. Humans would be much more resistant than bison to such attempts, given the much greater complexity of what we call "personality", and most nativist religious movements such as the one you describe do not last all that long in comparison. Which is not to say that there aren't
other genetic consequences to such insular reproductive practices. Unlike personality, many diseases and disorders are coded for by a single gene or a small cluster of genes, and in such cases intentional decisions about reproduction can have enormous impacts within a small breeding population.
Also, setting the genetic question aside, the phenomenon of a younger population frequently leaving upon coming-of-age is one we see in many disadvantaged communities around the world, and the social impacts of this trend are considerable; resource production can stagnate, cultural and linguistic transmission are badly interrupted, ecological relationships often become maladaptive, and the group becomes more vulnerable to external exploitation. On the other hand, research shows that the communities most affected by
human capital flight are in many ways benefited. Those who leave mostly stay gone, but not everyone stays gone
forever, nor is the economic (and sometimes legal) link between themselves and their families entirely severed; for an impoverished community, the presence of a missing generation can carry significant and even surprising economic and political advantages for those left behind, despite the often painful costs.
