The fact that people still vary on the extraversion continuum implies that natural selection could not deliver a clear verdict for extraversion over introversion.

The persistence of both introversion and extraversion in modern populations means that it is a balanced polymorphism. So the average benefits of being an introvert must be roughly equivalent to the average benefits of being an extravert.

The problem about this thesis is that the benefits of extraversion seem stronger. In particular, there is evidence that extroverts are better at handling stress, thereby conferring a probable advantage for psychological, and bodily health.

The assumption here seems to be that nature will select for only one "ideal" set of qualities for a member of species for a given environment, and that as a result, members of the species will be more and more alike, almost to the point of everyone being identical twins. But what you see instead is a variety, and populations which support a variety of traits are better able to adapt to changes in the environment, etc.

Maybe it's a bit like saying nature has been trying to figure out if men or women are more ideally suited for the environment. Obviously, a population is not going to survive without both. Perhaps it could be that human groups couldn't survive well if they had too many introverts, or too many extroverts.