I was wondering if Jinbe’s late addition to Straw Hat Pirates crew is not just strategic reason in terms of plot but also symbolic.
Luffy should always be the strongest Straw Hat being the leader. And as we know, Luffy has just learned how to use Advanced level of Armament Haki.
If Luffy hadn’t learn Advanced Armament yet and Jinbe joined before that, technically Jinbe would have been the strongest person in Straw Hats crew.
I think right now Luffy is the strongest, but he has surpassed Jinbe’s level only a few chapters ago. In my opinion Oda always wants Luffy to be the strongest Nakama. So, unless you still think right now Luffy is weaker than Jinbe, or that Jinbe was never stronger than him to begin with, all this makes perfect sense.
It should be noted that also Sanji and Zoro just got a power up, which can be said brought balance to add Jinbe to the Straw Hat Pirates crew.