The way I see it, if they made Archer into Ranger, what would be the point of having Musketeer and whatever it's advanced job would be down the road?

There would be two ranged DD classes unless they turned Muskteer into a support class, which just doesn't seem likely. Guns = hurt. (And Corsair wouldn't really make sense as the advanced job, since Musketeers are opposed to pirates.)

They're just turning it into a needed support class, and leaving the ranged DD slot open for Musketeer in the future so they don't have two classes that are basically the same with different weapons.