Actually that would be a pretty nifty way to extend the game by adding a new difficulty level--the 2nd time arpund you play without a death knight! It raises all kinds of philosophic questions too. For instance can good survive without the presence of evil, etc.
If it would allow you to play the game without the Deathknight for a second time, wouldn't it be practically impossible to undo the soulforging??? And, no, Good can't survive without the presence of evil, but then again that's why there is neutrality.