I definitely have thoughts to share-- but I should probably let them percolate a bit longer first.

I was a (seeming) extravert as a child, and I'm very much an introvert now... but I think that the difference is that in children, this personality difference seems to mostly be judged-- mistakenly-- on the basis of
gregariousness or
timidity/social anxiety. If you are gregarious and not timid, then you MUST be an extravert, yes? Aren't ALL assertive or charismatic people extraverts by nature??
Not so much, actually-- this set point really only authentically describes your recharging mechanism, at least in my understanding of things. So extraverts NEED other people to feel refreshed and energized... and introverts NEED time to themselves to do so.
I needed a lot of time with my own thoughts as a child
in spite of my talkativeness and social gregariousness. I still slip that persona
on and off to suit my surroundings, which fools many people in my life into not realizing that I'm a hard-core introvert. But I am. I get seriously unhinged after about 72 hours if I don't have at least a couple of hours of QUIET me-time each day, and at least half an hour or so needs to be continuous and uninterrupted. I'll gladly forgo sleep for that time, honestly-- I need it that much.
On the other hand, my DH is someone who is a true extravert in spite of being somewhat socially awkward with others, and easily frustrated in his communications with them. He gets unbearably lonely after a couple of days alone in the house. Me, not so much. I
like it.
I've had people close to me ask me which of my sides is "real." That has always struck me as peculiar. They
both are. I like people. I just don't NEED them. My father was a truly bimodal person-- he had
both needs. I've not known too many people that had extreme needs in both directions like that, but they must exist because he was one.
Introversion and extroversion are just differences in what constitutes a "healthy" or "toxic" lifestyle/environment/work-life balance for a particular individual. My DH has a lot of friends and spends MOST evenings away from home. I find that four scheduled evenings a week is
well out of my tolerance zone for people-intensive activities-- because it cuts into my down-time too much.
I have solitary hobbies-- meditative and quiet, most of them. I always have. It's not frustration with others, or a lack of enjoyment of them... it's about the primal need for me personally, which is introverted. For my DH, that primal drive is about social interaction.
It completely makes sense to me that in an evolutionary sense, introverts are less common than extraverts are-- primates ARE social/tribal creatures. It also makes sense, though, that introverts are evolutionarily necessary-- because we can be immune to groupthink and peer pressure in ways that extraverts struggle with.