Nah all Admirals since 800 years ago isn't going to be same level as Kong, Garp, C3 and The recent convicts.
It was obvious ..
You're probably right, the new cope should be they let Vice Admirals like Momousagi and Chaton of the past become Admirals, but after Xebec murked one at the reverie and The Great Pirate era's inception, they go above and beyond to make sure that never happens again. Which is why they let two randoms with no Navy experience become Admirals, they want apex fighters.
Maybe in a different era characters like Momousagi and Chaton could be Admirals. Also it was stated the Marines are stronger than ever right now, hard to believe that if Aramaki and Fujitora aren't the real deal and are supposed to be on par with this nameless Admiral.
Hero of the Marines and Fleet Admiral > mere Admirals, that simple
Sengoku wasn't Fleet Admiral it was Kong. It took Fleet Admiral Akainu 10 days extreme diff to defeat Admiral Aokiji.