Ehhh, Zoro is aiming to take down an emperor'a rival and warlord. Luffy is aiming for pirate king and has taken out 3 warlords on his own. WB and Shanks have admiral and warlord-level fighters in their crews. I don't see the big deal in Luffy getting Jinbe on his crew when you consider Luffy is aiming past WB and Shanks.
It's just that all the warlords up to now have been arc bosses, but Luffy:
1. already befriended the WB pirates
2. Teamed up with warlords
3. is siblings with the chief of staff of the Revolutionaries and WB's 3rd division commander, who was also offered a warlord position before he caught WB'S eye. Both siblings have aided and fought alongside Luffy already.
4. is going to be aided by the Hero of the Marines, Garp, who has to go through a redemption arc after hesitating to save Ace, and will die with a smile on his face, like all Ds do.
5. Has already befriended 4 additional Revolutionary commanders, either directly or through Sabo.
6. Has been aides by, and will be aided by, Dragon, the leader of the Revolutionaries, most wanted man in the world.
7. Is being bet on by Shanks, an emperor (WB, too)
8. Was trained by the 1st mate of Roger.
9. Demonstrated the ability to take out Doflamingo with one serious punch
Plus, Nami is arguably already the top Navigator in the world, and Usopp is on his way to being the best marksman. This crew is full of gifted and driven prodigies.