In one piece strength level is strongly tied to your genes, if you're parents are strong, then you're most likely going to be strong–might not be as strong as your parents, but strong nonetheless a.k.a Katakuri as an example. No matter how much the story tries to push the "will=Haki BS," the reality on the ground shows a completely different result. 99% of strong characters in OP come ether from strong parents, have a special bloodline (a.k.a D, Davy, Shanks' mother), or you're part of some royal bloodline like Sabo, Sanji, Zoro, Doflamingo, Oden, Loki etc. Plus the lab experiments, almost forgot about them.
The idea that characters become stronger by having tough fights is utter BS. All the strong characters are strong because of their genes, Luffy would be strong regardless whether he had his fights or was constantly trained by Garp. As matter of fact, he most likely would be stronger by now if he was forced to train under Garp (or even on the level of average marine recruiter) 24/7, just like during his time skip. I mean look at Kid, he doesn't lack the will or the hard fights, but yet he's not as strong as Luffy (or at least comparable to him) because he had no Rayleigh, Hyogoro or Gaban to train.
And yes, I know all about the narrative and perception bla bla bla... there's the narrative that the story is trying to push, and then there's the reality–no matter what Oda tries, he ends up with a character that has ether a unique bloodline, strong parents, OP DF, some Kind of experiment, unique weapon etc. Or all of them combined in one single character. The only character for now, that doesn't have ether of those things–ironically is Koby. His strength solely comes from his training ether by the marine instructors or Garp. And yes, I know that he disobeyed Akainu and showed some will power, but no matter how much the story tries to push Koby, he will never have the will power of even Ussop, let alone someone stronger. So no, I reject the notion that the story is pushing, strong characters are strong because they have good genes. If they're lucky, they'll get a good mentor, or they'll struggle like Kid.