Simon Cowell is not stupid - he would rather the X Factor go off our screens with some credibility left. Last year following the dive in ratings he was quoted as saying "I'd rather fall off the cliff on numbers than deteriorate slowly." and in March this year he said he would consider scrapping the show next year:
http://www.digitalspy.co.uk/tv/s103/...r-in-2013.html ...and with the 2012 ratings being even lower I believe it is definitely a possibility.
C5/Endemol, however, have no shame and will happily squeeze as much as they can out of Big Brother. I can honestly see them keeping it going - and if not normal BB then CBB - for a good few years yet. It will be in a pretty desperate state by that point but it could still definitely happen. Afterall they signed a two year deal after BB12 which was nearly down to 1m per night so clearly they're quite happy with what Big Brother is giving them.