I would be surprised if Marvin got fired. The fact the Bengals have made the playoffs two times during his tenure speaks volumes. before Lewis got there in 2003, the Bengals last went to the playoffs during the 1990 season.
Plus, after their D-coordinator Mike Zimmer lost his wife, and the death of Chris Henry, he managed to keep the team focused on the task at hand. I think he gets another shot next year. If he doesn't win next year, then he would be through.
Larry Johnson wouldn't have made a difference. Benson ran for over a 100 yards. It wasn't because Larry Johnson not playing that the Bengals lost.