You pretty much answered yourself in the first sentence. Rogue effectively is a thief class by a different name. Are there still differences in specialization? Sure, but enough differences to merit a whole new class or job? Not really. Could you force enough differences to make it possible (like with your "epitome of thieves" or "Phantom Thief" examples)? Again, sure, it's possible, but what's really the point? Whether you call it thief, ninja, rogue, or flibbertygargen, the point is that they're putting all of the thief-ish mechanics that they want into this one class. There's still technically room for a similar class, but there's much more creative maneuverability in keeping different jobs distinct. There are people like yourself that would enjoy a different type of thief/ninja class, but having an entirely different class (aka the gun class) will appeal to more people and allow for far more variety overall.