I can understand that Luffy has a ridiculous growth-rate, being able to jump from YC3 to YC1 within a day and go a step further by going from high-high tier to high top tier within a week or two, with most of the growth happening in the last battle. He is the main character. But if now everyone starts leveling up a tier to reach Yonko level, it makes you question how the Yonkos were undisputed for so many decades. Those are (mostly) natural born monsters with high level of Haki and monstrous DFs. And some WG guys are surpassing them within days. Especially since this is not the first WG generation.