If Nibali has a go today I'm not sure anyone from Sky can stop him. Probably the best descender in the TDF at the mo. Its a big gap to Sagan though.
As for Cav I'm not sure whats going on, I'm guessing he would have pulled out by now except for the fact that Sky have two guys leading the race. What I do think it shows is that despite his efforts he's not really climbing any better.