The etiquette of the race is that nobody attacks the yellow jersey on the final stage into Paris. I would imagine if someone was within a couple of seconds of Wiggins then it may happen, but the reality is Wiggo could have a lead of several minutes, especially if he wins today's time trial. And even if somebody wanted to try and attack tomorrow, the stage is relatively short at only 120km and Team Sky would not allow a potential winner, like Nibali to ride away from the peloton. The only thing that can stop Wiggo winning is if he falls off his bike and is unable to complete the race. Hope that explains it!