I also have doctrate degree and i was so lucky to have very very close friends in grad school. I didnt have close friends i would have failed! We shared notes , studied together.You are not taking into account the costs of being an extrovert.
My introversion got me my Ph.D. degree (as I could study all of my waking hours, unlike my competition), and my current career. Among other things.
In my class we had lots of extroverts. connecting with people is a key to success . My business is not growing now because i am introvert/shy. If i can get out of the shell i can make it grow big time. I really hate being an introvert/shy.
luckily my manager is extrovert and he deals with my staff. My point is please know that extreme introversion is not a good thing.
There is a saying" anything extreme becomes poison "