The way that the whole game is designed is to really require multiplayer interaction to get very far. I know from experience that to have a really good session requires a group of mixed classes where people know what their roles are. If you have players who aren't pulling their weight, it can be quite difficult at points!
IMHO, there's no way that SOE could translate that to an offline mode of play without seriously compromising the game. Whilst I recognise that the subscription can be offputting, I also see that SOE put a lot of work into constantly updating the game and it's content as well as organising special events within the game.