Except they can't remove the existing classes completely because the classes are fundamentally tied into not only the game's storyline but gameplay, it's not just a simple matter of changing a class's name to a Job from level 1.

Using your GLA example, the GLA's Guild is the Colosseum, which is a fundamental part of Ul'dah's story and lore. Just changing GLA to PLD would require a whole series of new quests would have to be written to 'explain' how a new adventurer is suddenly a 'free PLD' where as the game's storyline currently states outright that the skills of the Sultansworn are a tightly guarded secret and that they're only now passing on their knowledge to a select few veteran (not new) GLAs. Then there are all the enemies that have the class as well have to suddenly have their class name changed to reflect it for consistency, which again does not work when it's a Job that is supposedly rare (half the Jobs are outright stated as being forbidden or forgotten, like WHM). See my point? Pull one loose thread and the whole thing unravels, it's just not worth the effort on SE's part.

Hence, new Jobs added to the game will continue to be classless but the existing classes will have to remain as they're the Armoury System's very foundation (hence you won't see a FFXI-style system of Job and weapon type being separate and being able to equip multiple weapon types to a single Job).