To be fair, Greenbull could just be the jobber of the Admirals like Moriah is the jobber of the Warlords. Moriah being an embarrassment didn't invalidate the entirety of the Warlords. Likewise, we still have Akainu, Aokiji, Kizaru and Fujitora to potentially redeem the Admirals.
Regardless though, this does suck arse and I see no narrative purpose as to why Oda even bothered doing this to begin with.
Perhaps in the future, Chadmirals will shine again once more but as for the present however, nothing to do but go down with the ship.
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