The DM.
If the players want to give up fighting the Lich King and his Army of the Infinite Damned (or whatever), that's their prerogative, but later on when the ultimate conclusion is "Since you decided to go back to fighting kobolds and breeding yaks, the Lich King's army has swept across the realm and brings forth 1,000 years of darkness," none of the players can really complain, "But we decided not to do the lich thing anymore! Too much railroading, no thanks."
Whether the DM decides that other people in the game world are capable of rallying together and defeating the BBEG or that such a task could only be accomplished by the small group of especially lucky adventurers who have managed to accrue a great deal of strategic and personal resources, each approach is equally valid. If a player wants to leave it up to somebody else, he should probably find out whether there is anyone else in the game world capable of doing so before getting shirty with the DM and leaving the campaign.