If, in D:OS2, all available companions are those 3 that escape prison with me, I'd be pretty disappointed.
In theory, there could be safe havens in the world where you can meet other sourcerers, but they would need to be designed to fit the plot (potential replacements wouldn't be hanging around public places like bars or libraries).
That's the obvious problem.
Though I do wonder if the limitation (to a fixed set of non-changeable companions picked at game start _and_ to a party full of Sourcerers) is somewhat misguided.
Sure, for the MP case, 4 characters that all start in prison seems the right thing to do, i.e. one for each player. And no player would expect to switch his "main" character during the game, so no alternative companions needed. Also, every player probably wants/needs to make use of the cool Source talent, as it is inherent to the main plot that each player might persue on his own in competitive mode.
But for SP? Does my party really have to be all Sourcerers? Why not pick somebody up along the way that may be sympathetic to my cause? Or another that's not so sympathetic, but good at hiding his distaste and following his own agenda? There are practically limitless possibilities here.
Come to think of it, this seeming fixation on a full SP party escaping from prison grows to be my biggest gripe right now. Lets hope that this is given some more consideration further down development.