This sucks. Not to mention the timing is pretty bad. First, he won't cover the 2016 election and they'll still be breaking in the new person by then. Second, he leaves almost immediately after the two best options for replacing him left - Colbert and John Oliver. Maybe they can get Oliver back from HBO, but that show is killing it so maybe not.
You need someone funny but also sincere if the show is going to maintain its legitimacy - and it's hard to find comedians like that who can also carry a nightly show. They may have to dig into the younger/lesser known crowd to find someone.