I don't see why they'd change it.

Mighty Guard has its penalty because it's taking DPS job damage and turning it into Tank job damage.

Thus there's little reason to change Mighty Guard. It's not like it matters given that "BLU Tanking" is essentially just White Wind spam.

The skill is supposed to be a trade off, becoming a makeshift Tank at the cost of being a DPS. Rather than being a superfluous "Stance" that isn't used like with actual Tanks. If the 70% damage penalty was removed, then you'd just have all BLU's running around with Mighty Guard on all the time for free damage mitigation.