A lot of people think that Old Legends from the previous era like Garp and Sengoku in their prime were stronger than current Admirals.
But I think that would really undercut Luffy’s achievements of becoming Pirate King if all the current Admirals aren’t as strong as the previous generation in their prime.
In the next era when Luffy is Pirate King I’m sure someone like Akainu will be viewed in the same respect as Garp is in the current era. What people will know is that he killed the brother of the Pirate King before his eyes and made him flee for his life.
So actions in the past are always viewed how it happened in that moment. We will know that at the end Luffy will probably beat Akainu, but the other events still happened.
So now look at what we know about Garp. 38 years ago, he earned his epithet “Hero of the Marines” during the time known as the “Era of Rocks” by defeating Rocks D. Xebec in God Valley, through joint forces with Gol D. Roger.
He is known for cornering and fighting Gol D. Roger many times. Roger also stated that he and Garp had almost killed each other on numerous occasions.
I think that the strongest characters of the current generation are at the same level of the Legends from the past generation in their prime, if not, Luffy becoming Pirate King will be less impressive than when Roger did it.
*Theory by KayWWW
