@xxjoshxx:
They have crews and allies that could break them out of Impel down no sweat, unless you want to send an Admiral or two to guard them and it all doesn't seem like the most effective thing to do.
I agree with you that holding a yonkou would be too much of a risk.
One admiral wouldn't be enough to guard a yonkou. Admirals are very tough but they are still human, he would have to take breaks to sleep, bathroom ect. So it would take 2 admirals to hold a yonkou that wasn't a willing prisioner (added that due to Roger, though not a yonkou, he was pirate king and equal to Whitebeard).
Add to that, one of the duties of the admirals is to the World Nobles, to protect them … and if a WN gets pissy at a pirate at Saboady, they call an admiral to off the pirate.
I doubt the WNs would agree to have 2 admirals tied up for a large amount of time and not be there to 'protect' them. One yes, but two? That would leave only 1 admiral to take care of everything else. If it was only for a week? yes I can see 2 guarding a yonkou, but over that amount?
Also keep in mind this is an Aikainu run Marine force now, I pretty sure under his authority all Yonkou would either be killed on the spot or executed on a later date.
Hmm that brings up a interesting thought. I would think that the Yonkou would be killed on the spot due to what trouble it would cause if they have an another war. As you pointed out, the youkou's crew and allies to come their leader's side. Not just if they were sent to prison but if they were to be executed at a later date.
At first I thought the WG and Marines wouldn't want to risk such a war. I mean its one thing to plan to execute a commander of a yonkou's crew, its a different thing altogether to try to execute a younkou. As shown in the Marineford war, the leaders usually stay in the background as much as possible giving orders to their officers and grunts. This isn't new its what done in real warfare.
But to have a yonkou, themself, its too much of a risk since they are so powerful. There's too much of a risk of them breaking free and killing much of the marine force. As Whitebeard showed, even though he was in his old age and ill-health, only 4 people at that war had any potential to oppose him = the 3 Admirals and the Fleet Admiral.
On the other side of the argument is that if they did somehow manage to subdue a yonkou long enough to execute him later, that would a big … really, really huge!... positive image boost for the Marines among the civilians. Enough that they would be tempted.