Then that's even more bad. Cause, none of the kuja top tiers were fighting. And it literally took the Marines DAYS to break through fodder.
We have no idea why coby wasn't affected lol. It sure as hell wasn't haki. Then why didn't oda just show that?
BB said he was killing her despite wanting her fruit, because he understood that she would turn him to stone otherwise. He was willing to even let 2 of his commanders stay as stones forever.
Wait until we learn about Rocky port before getting your hope too high.
The thing is, Coby hasn't shown anything exceptional at all. He wasn't the Leader in that invasion. He didn't get what he wanted. And he got kidnapped as a consolation price to be made damsel in distress. He is now at the mercy of the Black beard pirates. He wont be escaping on his own. So he won't be able to train and get stronger either. And he is not the Garp parallel. Smoker is.
The only thing coby has is his promise to luffy about becoming an admiral. But thats a very vague promise. It might not come to fruition, and even if it does, it has more chances of it being an epilogue thing rather than in the current story.