Different eras called for different policies.
When Kong and Sengoku served as Fleet Admiral, the three Admiral positions remained vacant for quite a long time, and the degree of danger on the seas back then was nowhere near its current level.
Pirates still caused trouble, but the Four Emperors, the Revolutionary Army, the Worst Generation, and all those chaotic elements did not exist yet.
However, things were different now. At this point in time, word of Ryokugyu's mysterious demise was disadvantageous news no matter how one looked at it, which led to the outcome where Ryokugyu experienced a "mysterious ascension" and Chaton temporarily filled the vacancy.
After all, he was an Admiral Candidate, and when there truly were no ready-made personnel available, it fell to them to play their part.
As for carrying out missions, there would naturally be some compromises made; it was the only possible way, and as long as Aokiji knew how to assign tasks, no major problems would arise for a while.
The outside world had no way of knowing the depth of these matters. In the public eye, Ryokugyu had previously belonged to the ranks of nobodies as well, and the identity of an Admiral was in itself the greatest gold-plated signboard.
Before a true top-tier showdown occurred, very few people were willing to stick their necks out as the first bird, and the logic that allowed Buggy to be dubbed a new "Emperor of the Sea" by Morgans relied on this exact same thing.
Heaven-defying luck combined with a bit of overthinking from his subordinates meant that no one was willing to risk their lives to test this "strongman's" edge, ultimately causing Buggy's fame to grow greater and greater.
Moreover, when Crocodile took people to find Moria playing dead, the three former Shichibukai ended