Disclaimer: This thread is not intended to be anything profound. Rather, I want to highlight something intuitive that I hadn't given much thought until recently.
I've seen a fair amount of people clinging onto the old gen top tiers even after Imu's full reveal. I'm not sure if this is about resistance to power-creep or simply a deep love for the OGs. But I believe that, compared to other prominent shonen, One Piece's power-creep can actually be rather smooth. I say this as someone who thinks that current One Piece is dogshit and Oda is so washed that your local laundromat is asking him for his secret.
Anyway, the main point:
What are the main ways that someone becomes a competent fighter in One Piece? Aside from innate abilities (including but not limited to Devil Fruits and race-based powers), I would say that training, knowledge and experience account for the vast majority of one's power level.
We know that the Ancient Kingdom was so advanced that even Vegapunk had trouble understanding their creations. This implies that they had access to a wealth of information that is probably unquantifiable. How is this relevant? Because it is highly likely that this included a much deeper, much more comprehensive understanding of both haki and Devil Fruits, and potentially other unexplored abilities (for example, Maki/Omen). As a result, the fighters at the time would have been significantly more knowledgeable than even the old gen top tiers like Garp, Ray and Gaban who seem to know everything about combat.
On top of this, we know for a fact that the technology present in the Ancient Kingdom make the modern ones look like child's play. Between the Ancient Weapons, Emeth, and now the legendary weapons wielded by the 20 Kings, we've seen firsthand how this technological sophistication bleeds directly into warfare. Imagine how strong you would've needed to be in order to compete with stuff like this. If anything, Imu's strength was a necessary adaptation.
When you put all of this together, it is very easy to see why Imu is as strong as he is. His contemporaries had the best technology, the best understanding of haki and Devil Fruits, the best weapons, and consequently the best overall skills - of all time. Furthermore, we know that haki blooms in the heat of battle (i.e., experience makes you a LOT stronger). Imu, Joyboy, the 19 Kings, and all the top tiers of their era - fought in the biggest war ever. They already had all the ingredients to become God tiers compared to the current era, but when you throw in the experience card, it reinforces this idea even more.
However, everything from that era was utterly erased. All the technology, all the knowledge, all the weapons -- completely buried and eventually forgotten. So what does this mean? It means that for the last 800 (900?) years, the top tiers have been playing catch-up. They are all essentially like primitive cavemen to someone like Imu, despite existing chronologically later. Their ceilings are capped because of their limited access to information.
Once Luffy and *his* contemporaries rediscover everything the Ancient Kingdom knew and had access to, they will be able to surpass everyone who came before them, by a country mile. And it will quite honestly be logically justifiable.
TLDR:
The powercreep makes sense because Imu comes from an era marked by the absolute pinnacle of knowledge as well as the pinnacle of fighting experience (war). Knowledge translates directly into fighting prowess (technology, optimization of abilities, etc.) while experience is the quickest way to improve, so Imu's power-level is understandable. The current gen has significantly less knowledge and experience in comparison to the point that they are fairly primitive. When Luffy's gen gains all the knowledge and war experience of the Ancient Kingdom, they will be able to catch up to Imu's gen.
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