They wouldn't want to. Especially Garp I'd think.
They could still fight the crap out of 80% of the pirates out there of they want to, but they don't.
After the war they felt stressed out and wanted to go on retirement because they didn't think they'd fit into the new age anyways. They've been around for long enough.
!
I realised my error in that Bonney wasn't introduced/in Marine hands at this point. But mainly i was talking about above point
(sides who wants to grow old anyway:ninja:)