I can't see how money was ever the motivator. George was hardly on the bones of his arse when he came around to making the prequels. And he personally financed all of the films himself so he'd have to have an awful lot of confidence to think he was going to make a ton of money off them.
I don't know if Titanic really justifies Cameron as a visionary director. I really don't see any boundaries being pushed in that film. Avatar, yes; Terminator 2, most definitely.