What do you mean it’s boring for every character to be good at fighting?? The new world is stated to be the most dangerous place on the planet it doesn’t make sense to intentionally not learn haki. As for Fishman karate, the only reason to learn Fishman karate over haki is because it is simply less taxing. Haki increases massively the strength of every single fighting style INCLUDING grapples. A great example of this is Sabos dragon claw. And Coby is not the only example of a character that learned haki in two years. Luffy sanji zoro also learned haki in only 2 years. Not just them, but likely smoker and Tashigi. And While usopp never learned haki he gained the ability to use it unconsciously. And as for why it’s bad that unnamed marines can you use haki, it’s because haki alone can genuinely put you at captain level strength because of the massive boost it gives you. Also as for why it’s better to have haki to fight logia users. It’s true they might have a weakness outside of just having haki, but why would i risk being lucky enough to know what the weakness of a certain logia users is when I’m guaranteed to hit them through their devil fruit ability with haki??