Maybe in Future they will add THF to Rogue , so it will be like Arcanist = sch+smn . Rogue 1st of all it's a class and not job , like Gladiator for PLD , and Thief is actually Job - not a class . So that why they actually able to add it in future . Also Yoshi said Thief guild will not fit current Eorzea , but Job quests we actually take from certain "NPC" who in many cases WAS use job (Exmample Bard , or currently on job while other "citizens of Eorzea not . Adding some lore + some "Old man" NPC who before was greatest Thief may fix the problem
sorry for my eng , was sleepy when spotted the topic