Character consistency, for God's sake, it would make no sense for someone like Jinbe to not stay behind and protect his friends. Not everything needs to be explained by some intrincate plot twist or big consequence in upcoming events. Sometimes it is just a matter of quality writing that actually respects the logic behind the characters.
If you want some additional explanation, odds are Luffy would have never fallen from the ship when entering Wano because Jinbe is an exceptional helmsman that wouldn't be tricked by some whirlpools and waterfalls. And even if he had fallen, Jinbe would have rescued him ipso facto (so no Tama plot nor separation). But this isn't even required because Jinbe staying behind was the most logical thing to happen regarding his personality.