You're extremely fixated on crew posts. When was the need for a Sniper, Archaeologist or Helmsman mentioned before Usopp, Robin and Jinbe joined? The answer is never, and all 3 displayed abilities for their role that only they are capable of doing.
What lookout ability has Carrot displayed that only she can do? Oda went out of his way to make Jinbe'a helsmanship unique. Where is Carrot's who's been around for over 200 chapters?
Vivi and Robin has relevance and importance to the plot, and wasn't sitting in the background getting off screened while the SHs fought for their lives.
Yes I am concentrating on the post because ALL the storytelling for a new strawhat revolves around their post.
As I said the need/want can be either for a post, a character, or a particular individual . This is a
factual pattern
The need for a sniper was not introduced in the story but the need for "a new man" was introced just before debarking on Usopp's island.
The need for an archeologist was not introduced in the story, but the need for Robin was during a whole arc.
The need for an helmman has been introduced very subtly around whole cake.
Finally, the argument that a character is better than another to a post is meaningless as other characters can ALSO do what all the strawhats are doing. you REALLY need to listen to this argument (i've think i've repeated it 10 times by now):
What count is not what said strawhat can do all the time, it's what said strawhat can do in a SPECIFIC situation
Brook can be at the helm, but he can't enter the green room
Nami can cook, but she can't cook a wedding cake like Sanji
Franky can shot, but he does not have the accuracy of Usopp in dire situation
Each strawhats are serious but when it comes to the real deal only Zoro has the corones to keep everyone accountable.
All the strawhat are strong, but only Luffy can beat the bad guy
Etc.
For Carrot, Usopp/chopper and Robin can be look outs. but none has the experience, the body or the rapidity to go high fast, check the ennemy, take charge and cut their head off.