You made the terrible strawman, and now you are moving the goal posts.

Anyone who has children knows that they are born with their own little personalities and that - aside from drastic physical interventions - that little personality never changes. Society largely slides off them like water off a ducks's back. Because your personality is a genetic endowment.

By your own logic, social isolation should have almost no effect on their personality.

Well, it does. Your statement is wildly off base. You don't even need physical alterations to see personality changes.

I don't know if you are just very young, or very bitter, but personality changes massively over your life. You might be generally pre-disposed to some general trend or another, but your personality will swing as you age from all of the experiences you will have in your life.

Hell, I know people who's personality changed from diet. Social development is way more impactful than any of that.

Hell, this is an experiment that you can do with animals. Not that you really need one, because you'll see it all the time anyway. If you keep a horse in their stall for too long and they don't socialize you'll see significant behavioral changes, including increased aggression, defensiveness, fear, and violence.