They could easily keep her around or cast her off early, really her job in the books is done after the first one.Well, the thing is I guess, is that they've got a cast of lesser known actors and splashed out on a major league one, they need to give her some serious screen time.
My wife, who hasn't read the books, predicted that Moiraine will not last very long simply because Rosamund Pike isn't going to want to be pinned down for 13 books worth of TV fantasy potboiler. One might note the example already set by Snowpiercer in this regard.