Robin is not the only serious-toned person in the series, she is not the olny one who does not have any goofy expression like Zoro, Sanji, and Nami. I've seen many people in the series like that, I mean look at Kohza, Makino, Wapier, Lucci, Arlong, Kuro, Crocodile, Dalton, Ohm, Shura, etc, etc, etc.
Robin is fine the way she is, she does not have to have a sharp teeth and a missing eyeball when she is angery.
There is nothing wrong with her being quite, that what makes her think clear, she olny talks when something intersting happens, she does help people(Remamber she saved Aisa and Parrie from falling), She stole the lock post from the Monkeys, she helps Chopper and Usopp, she gives nami the treasure she stole from Crocodile, she gives Luffy her advice.
She is the brain of the group and she always thinks frist. She is friendly, just shy. She is subtle for a good reason, she is not a fast-thinker and she is very mature.
That what makes her unique, each crew needs to have differance kinds of personally, not just bunch of goofy people like Luffy, Usopp, Chopper and Sanji. She does have personally, she does have emotion(Non-goofy emotions), she does have feelings, she is not a hollow shell of a woman or a old nagging lady.
If all crews are goofy, it'll be no fun and not intersting, there has to be someone serious like Kohza to balance the crew and Robin is the answer. Robin reads the phonglyph and history, she does have dreams and that is searching for the real history and she have a trumaic past theofore she is human not a demon in disguise.
So what if she is 28 years old, she does not have to be a teenager. So she fits the crew, she is what makes the story intersting, without her One Piece will suck MAJOR assball.
Usopp don't need to leave, we need weak crews to balance out strong crews and he also makes the crew intersting.