Nothing will block him from coming back if he doesn‘t die ,this isn‘t like Snack who got demoted after losing.
It‘s not like he will vanish
All Vilians who didn‘t die came back in the story at one point or another .
and Btw Marine HQ had for a short amount of time only 1 Admiral after Kuzan and Akainu fought .
So if there where 2 Admirals for a short amount of time it would only be a matter of time until someone takes the seat ,if an Admiral dies which i doubt.
So you think all 3 would lose collectively in one fight or war ,Marineford 2.0 but this time Admirals lose?
Or how do you see the whole Marines go down in one fight?
This isn‘t like any other organisation where they are in one place ,they are worldwide .
i think Marines won‘t go down as a whole