Nothing will "replace" Lost, and, quite frankly, it's foolish to pretend that anything will. The show is a once-in-a-lifetime achievement, and, in keeping with that notion, I don't expect to see anything like it again. 24, on the other hand, may have been ahead of its time and a groundbreaking achievement, but that was more than five years ago, and it's been showing signs of wear since the fifth season.
Next fall, Survivor will be all that remains of my once-dominant trio of programs. Other contenders, long waiting in the wings, will need to step forward - Friday Night Lights, The Office, or Chuck, for example. Will they be the same? Unlikely, but maybe something new fills the void. I can only hope...