Maybe the rest of the entire crew, including new members, will learn observation Haki to make Usopp the weakest haki user? That organizes everything, and we can figure out which members will learn CoA haki.
However, when the Logia member joins, there's the potential that members can be stronger than the Logia user, other than Zoro, Sanji, and Jinbe. I'd argue one more member of the straw hats will learn CoA haki.
The Logia user themselves who would eventually join the straw hats possibly using CoA haki would not be as necessary as having CoO to be stronger than Usopp. Someone (not Sanji) might use only CoO, and CoC, meaning they can't defeat a Logia user.
Sanji unlocking CoC would make him stronger than Jinbe, but cmon.