I have an impression that (if Mighty Guard would still exist for 5.1) they'd likely remove only the damage penalty from it, otherwise they'd need to have a REALLY good excuse to explain why it should stay there.
If I were in the devs' shoes for Blue Mage though, unless they were going to make a Blue Mage/Limited Job-only duty roulette, I'd likely scrap Mighty Guard completely and buff any defensive spells to compensate.