Well it just goes to show you how weak the pool is. I mean legitimate top tiers are pretty rare, it's quite an accomplishment to reach top commander level in this verse.
Remember how Marco was a Yonkou candidate? I'd figure the same goes for the Admiral candidates, they prob cap out around this level.
Which is why Akainu would rather hold a draft to hire and promote two randoms with no Marine background, to such an influential position in the entire Navy. Shows you how desperate they were, if the Admiral candidates were anywhere near top tiers, them having the Marine background would be more than enough to just apoint them in the position, atleast temporarily.