My issue isn't with the fact we have WG antagonists who might be stronger than admirals. It's the way the story was set up.
-The WG is terrified of the yonko. The Gorosei were worried about making Kaido angry and even Admirals like Greenbull openly admit to fearing him (along with Shanks). The Marines wanted every advantage they could get in order to go to war with WB, including brainwashing Squard to sneak stab him even though WB is old as hell and sick. The WG/Marines always tip toe on eggshells with the yonko. Nobody dared to invade Wano or Whole Cake. Nobody dared to try and take back Fishmen Island when Whitebeard said it was his territory. Nobody dared to start a war with Shanks when he challenged the Marines and pirates at Marineford. The World Government had to make deals with Kaido and Orochi for weapons. None of this would make any goddamn sense if this whole time the WG has had multiple fighters who are yonko level or above just sitting around doing nothing.
-Garp stated that the WG created the warlord system in order to maintain the "balance of power". The Marines simply could not handle fighting all the pirates themselves and allied themselves with other notable pirates such as Mihawk, Crocodile, Doflamingo, etc so that the yonko didn't just totally dominate the Grand Line. This is actually similar to governments in the past using buccaneers and privateers.
-The warlord system wasn't removed until AFTER the creation of the serpahim despite the fact the warlords became a major liability and double crossed them several times. Again, the point must be emphasized that the Marines COULDN'T HANDLE the pirates alone.
-The Marines were desperate for more strength after Aokiji quit and needed a world draft to enlist Greenbull and Fujitora. The implication is that fighters who had Admiral level strength were rare and that the Marines simply didn't have enough top tiers to fill their own ranks. Even the Vice Admirals simply didn't measure up outside of outliers like Garp.
-The WG tried to end the great age of piracy TWICE. The first time was by publicly executing Roger, they hoped they could kill the motivation to go out and plunder but this backfired. The 2nd time was by killing WB but again, WB managed to keep the burning passion of piracy alive by confirming the One Piece exists. Both times the WG was extremely angry and they desperately wanted to end the surge of pirates.
Put all these factors together and the picture becomes clear: the Marines sorely lack manpower. They need to rely on sheer numbers and outside forces such as the warlords/serpahim to keep up with the pirates. The WG itself is scared of the yonko and the yonko are >/= Admirals in individual strength. It wouldn't make sense if a bunch of Admiral+ level characters are just sitting around at Marijoa doing nothing when they could have been helping the Marines put an end to the great age of pirates. It's nothing more than plot induced stupidity.
Again the issue isn't the possibility of characters being stronger than admirals...I don't care if that's true. My issue is the writing makes the reveal very heavy handed and confusing. Why the hell would the holy knights and gorosei just chill on the red line when Akainu desperately needs more manpower?!? Why bother teaming up with warlords if you could have just had the holy knights help patrol the grand line instead? Similarly, why the need for a major world draft to find soldiers with high potential like Greenbull/Fujitora if you could have simply had some holy knights become Admirals...??? Why waste time with this big stalemate with the yonko system, making weapons trades with Kaido, negotiating with Shanks, etc when you can just gather all the Gorosei+Admirals+a huge fleet of ships and pacifistas to storm a yonko territory and wipe them out??? No yonko would survive that. The Great Age of Piracy would be OVER...! The grand line would belong to the World Government!
You are getting the World Government (WG) and Marines/Navy mixed up. The Marines and Navy serve the World Government, but they aren't exactly the same thing. In comparison, the WG is basically the United States, and the Navy is the U.S. Navy. They are related and connected but aren't the same thing. Especially in a world like One Piece where the World Government has its own secret fighters.
The Marines/Navy and Admiral's role is to counteract the Pirates and Yonko. The existence of pirates justifies the World Government and Marines existing to the average man in a World Government-aligned nation. justifies the heavenly tribute and celestial dragon's status as the "gods" that created the One-Piece World Government. The World Government is a shadowy and mysterious organization that doesn't show its hand and abilities until absolutely necessary.
TBH, I don't think the World Government cares about taking over places like Wano, Whole Cake Island, Fishman Island, and Elbaf. I believe that the WG is satisfied with the current number of members and doesn't want such strange people being added to its protection and service. The WG doesn't want to expend its resources trying to wipe out the Yonko went they can make deals with them or just outright ignore them entirely.
Big Mom, Kaido, Shanks, Whitebeard, and Roger's goal wasn't to overthrow and destroy the World Government by themselves like Monkey D. Dragon is trying to do. The Yonko don't like the World Government and Marines especially; however, they aren't trying to overthrow the entire world order and society because it is too dangerous and costly. Basically, the Pirates and Yonko and Marines and Admirals are in a Cold War Superpower tug of war. Two powerful entities that don't want to step on the other's feet.
The World Government doesn't truly care about eradicating all piracy at the cost own their own regime and massive internal casualties. Imu, Gorosei, and Holy Knights don't care about wiping out the Yonko unless they attack the celestial dragons and Holy Land of Mary Goeis itself. It is the Marines and Navy's job to be focused on killing and wiping out pirates and providing justification for the Marines and the World Government. The Celestial Dragons don't feel threatened by the Yonko or Pirate King unlike Rocks D. Xebec because unlike them he wanted to become the King of the World and possibly overthrow Saint Imu-sama of House Nerona just like Monkey D. Dragon. Hence, why Xebec committed the World's Greatest Taboo and Dragon is the World's Most Wanted Criminal.
Roger, Whitebeard, Big Mom, Kaido, and Shanks don't want to overthrow the World Government but create their own spheres of influence in parts of the One-Piece world. Only, the Sun God Nika can bring the Drums of Liberation to Mary Goeis! JoyBoy tried once before eight hundred years ago but failed even with allies such as the ancient kingdoms of Skypeia, Fishman Island, Alabasta, Wano, and etc but still lost to Imu and the other twenty founders. It is Luffy's destiny as the Bringer of the Dawn, Reincarnation of the Sun God, and Holder of the Will of D to fulfill JoyBoy's role!
