My thing is admirals are like the perfect op characters. They fit the tone perfectly. Which is ultimately a funny silly adventure story about people eating crazy fruits with powers and exploring the world, with a dark side at the same time. Its also just kind of refrences to various mythologies or things in pop culture,which is why they're all based on actors oda likes for example. We see them interact with eachother,we see them all deal with the consequences of the fucking evil regime they prop up, while seeing the consequences this leads to them.(ofc,Oda has kind of ruined this with fake out deaths, but I guess the fact that they even tried to kill their close friends is pretty heavy). Garp and Coby are just kind of meme characters imo. They don't really deal with the consequences of propping up an evil regime( obviously Garp refusing a promotion to an admiral is just a dumb excuse as well as him supposedly privately hating the celestials or whatever. Same with sword, who are basically just marines but they say they quit. Which is dumb imo) they're kind of just called heroes by the world, for reasons that don't even really make sense/aren't explained. And yes I'd say them not having fruits is part of the problem. Instead of giving them powers that are really interesting to watch in action, they just kind of punch hard,and sometimes they punch harder because they were born with conquerors haki(despite them not conquering anything ever).