They won't introduce a new race because, as you say, they'd have to offer race changes to all existing players, which would cause a massive inbalance in FFXI's racial demographic (i.e. like 40% or more characters would be running around as this new race, Manthra or whatever it happened to be, unless it sucked).
Besides, what new race could they possibly introduce? We have the mage race, the melee race, the brick wall race, the agile/dextrous race and the balanced race already. And they won't add Manthra, because one of their fundamental characteristics is being extremely rare (it's even woven into the storyline).
XI Wiki



