@Nolus:
It's true that it's not a "must" to have 3 Admirals, but now, being a marine is harder than ever. WB made sure more pirates will set sail before he died, the marines suffered great losses in the war…
Maybe two Admirals aren't enough. Plus it'd make people feel a little more comfortable knowing that there are still many powerful marines out there, like: "Have you heard? Vice-Admiral "insert name here" got promoted to an Admiral! He/she must be really powerful! I'm glad he/she's on our side!"
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You should search for the manga page, where it's said.
We know that VAs can be in charge of bases (Komir- G-2; Momonga- G-1), but I'm not sure about the Admirals. Sure if they want they can order hundreds of people around, but I don't think they're the ones fully responsible for all those marines. "Taking care" of that many people is not that easy.
Maybe it's like as you said it,but i am talking about us,the readers.At least for me it would be strange after so many years of "hyping" the Admirals and so many years of the VA being nothing more than just a "rank" suddenly one of them becoming Admiral.I mean if the VA,even 2-3 of them,were such a big deal the shichibukai would not be needed.My point is if PRE TS Luffy managed to take down 2 of the Shichib,and knowing that the VA are nowhere near their strenght it would be strange,for me,to see any of them as a new Admiral.
I would prefer to be 2 Admirals rather a VA getting promotion or a completely new char.But maybe Oda will come with something like,"a really strong character stays always in the NW to keep an eye,and that's why he didn't show until now in the series,in the war etc etc…Afterall who would have thought that someone like Magellan would "pop out" out of nowhere?
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@akagami7:
I remember have read that each admiral controlled a third of the marine, bu i can't remember where i read it, any thoughts on this?
I never read anything like that,i think you are a bit confused :)