I think, from my understanding, Garp chose to stay in the Marines because its importance to the world order and wants to change it from the inside. He hopes to pass on the torch to his family but it ultimately goes to Coby, who has a perfect sense of justice.
She already crossed that line through her "inventions", governing policies & system; as well as winning a what's supposed to be a historic loss for Oda and significantly improved his image among the citizens.