They can't just 'claim' to be unfit. They need to be examined. The WC organisers would sue the French FA for loss of earnings, having to refund money to punters, potential sponsor money lost. France would also be banned from the next WC and the French FA would lose all the potential income from that. They could sue the players for an enormous amount of money.
It absolutely won't happen. The players will be told by someone what the potential consequences are of breaching their contracts, and they will turn up. Whether or not they will try is another matter.
Of course, ironically, if they were foolish enough to refuse to play, the one player who couldn't be sued would be Anelka.
And the sums would be enormous. Look at Adrian Mutu.