Another aspect is, why do people think Luffy and Zoro have trained with the best in the world in the first place?
You can argue that the other Strawhats needed powerups for the New World and they were not ready, compared to L+Z but that was not the issue.
The issue was, Luffy´s aim from the get-go was to target the elite of the world, and he is no schemer or someone who patiently waits for any chance. So the aim of the training was, to put Luffy and Zoro in a position in which through those battles they would eventually enter, they could learn the necessary abilities to compete on this level.
Admittedly, Luffy learning FS in the middle of a fight is a stretch, but it´s still something built up based on his training for 2 years.
You can argue Zoro mastering Enma in 2 weeks is a stretch when none since Oden was able to do so, but Zoro´s training with Mihawk, and we literally saw him mentioning black blades, put him in a position to be able to do it.
This is not Dragonball, which lacked any transparency anyway and was only punches and energy blasts for the most part, and Naruto, which pulled one ability after the next out of thin air.
Having said that, Impel Down and Marineford is guilty of power creeping, Impel Down only one instant (Luffy punching Blackbeard) and Marineford a lot (Admirals, Vice Admirals, Sengoku, all have capacity to oneshot Luffy, especially in the state he was), so One Piece is not completely free from it (also why for me Marineford is one of the worst arcs, despite Whitebeard, Akainu, Ace death and so forth), but the events now? No, perfectly consistent and arguably even a good solution, meaning Kaido having an ability that protects him against physical damage without AdvCoA.