See, but here's a thing. He can enlist said guy without making him an Admiral. After all, he is under no obligation to make someone basically one of his second-in-command.
That's why I think he would take the powerful guy, but make him some lower rank, so that he gathers necessery experience. For some reason every Admiral so far is in his fifties, so I think experience does play a part in it.
Just my opinion, of course.
The conscription seems to break the kind of rules you need for usual advancement though.
It seems to be about recruiting guys that wouldn't have joined the marines otherwise, Fuji is at an advanced age, yet it seems he was never interested in joining the marines before becoming an admiral during the timeskip.
Guys who have the power needed to become an admiral but aren't interested in taking orders, or waiting for regular advancement could really be helped by the conscription.