Do you understand the concept of development inertia ?
Do you mean the concept you made up to justify your expectations as admitted in the very beginning of your post?
We will analyse in detail a storytelling concept I call "development inertia" (don't bother its a neologism of mine that I created) to see if Oda has indeed made a mistake with Carrot or not.
Yes, I understand the concept, I read your post. Oda is so great he incorporated your concept and then subverted it.
On a serious note, sure, Carrot wants to sail and find adventure, literally just like Vivi felt at home with the Straw Hats and wanted to go on with them to continue living adventures. Will they be back in the story in some form? Sure, just like Vivi has. Even Jyabura and Wapol have. She will help from the sidelines, or support, during the "final war" or whatever. Not much different to what she did throughout the story so far.
So what I mean to say, in fewer words, is:
He made a big one and it started with N.
This was just a personal unmet expectation you had. No matter how you twist it, it boils down to just that.