Action combat would be impossible due to server lag. If the game was built to allow clients to communicate peer to peer in battle, it might be possible to handle real time action, but then players are at the mercy of their network connection far more than they are now. With the polling interval in FFXIV and the 2.5s GCD, the game buffers itself against the impact of network lag to a degree. Also, at lvl 60, your GCD is significantly lower than it was when you were level 1, and it will continue to get smaller.