Leonard Barden wrote:Fide's published match regulations don't allow for a default:
3.6.2 If a player refuses to participate in the World Championship Match, he will be replaced
as follows: The runner up finalist of FWCM 2013, GM Viswanathan Anand, replaces the
World champion Magnus Carlsen and the runner up of the Candidates Tournament 2014
replaces the challenger. In case any or both players refuse to participate when invited, or for
any further replacements needed, the FIDE rating list of January 2014 will be used to
determine their replacements.
So if this happens, then effectively, Anand has loaned the title to Carlsen for one year, especially if he succeeds in beating Karjakin (I believe he is the replacement).
As for the players refusing to take part and using the January list for their replacements: well, what happens if there is a real boycott from the world's best players? (i.e. following Carlsen)
Is a world title match between Lewis Martin and Roger De Coverly on the cards?