I think the real issue is that PvP wasn't even a thought when the battle system was designed. It was shoehorned in after the fact. They've done what they could with the framework they had to work with, but the system just isn't optimized for PvP. The GCD is too slow, movement speed is too slow, there's not an adequate cc balance, class balance just doesn't fit, the format of the Holy trinity that ffxiv runs on doesn't lend itself to Pvp etc. It'd take a complete rework from the ground up that had both pve and Pvp in mind to put it on par with other mmos.