People that have datamined the game have found companion strings for 8 more characters.
It would be a very poor decision to limit new companions to just the first act.
In fact in real life, nobody has to commit to companions, and you always meet new people later in life. You don't have to commit to anything actually.
That reasoning seems very weak, and makes it seem as though the people that are making this game have never once in their whole lives stepped outside of their mother's basements.
