Garp isn't the only one that ran Marines, why'd anyone with a double digit IQ assume only Coby's gonna be the future of Marines? Lmao that shit doesn't even make any sense.
Garp never ran the marines because he was not fleet admiral. And because the marines are truly ran by the world government.
So essentially garo wants kobys purpose and role to surpass even garps. Since for koby to run the marines he has to be fleet admiral with a new revised world government