You're basically bringing up the infamy argument like so many others before you. Zoro was once more famous than Luffy and people for a time feared him more than Luffy.
And I will use Robin as a counter example but not with Luffy, with Zoro once again. People are always so focused on the specific balance and placement of the crews bounties they forget that Robin had the 2nd highest bounty for a heck of a long time.
Zoro is not the captain, I'm sorry but this is a weak argument.
If Jinbe had a bigger bounty than Luffy for a time then so be it. I feel like thats one of the weakest arguments people could bring up.
keeping the highest bounty on Luffy, the captain, which has been the case since the start of the series and the case as well with every other crews is not a weak argument.
The main antagonist after Fishman Island was Doflamingo. Once we learned that I can assure that was on of the biggest reasons Jinbe had not joined yet. You can bet Oda didn't want both 2 War Lords teaming up with Luffy to take down 1.
Or that Jinbe was too strong to be present at Dressrosa. Once again an argument used during Fishman Island. People who said that Jinbe could not join at the time because he was too strong were right.
Still despite all that, once more, just about anytime someone claimed Jinbe would not join it was always a reasoning for why he would never ever join the crew.
That's because no one thought about a Jinbe joining after his own arc and it never happened before. It was Jinbe joining at the end of Fishman Island or never, because it made sense.