Indeed, I think regardless of his sex scandal, Edwards would have lost. However, the scandal was just the icing on the cake.
John Edwards would've been decisively elected. The White House was clearly going to flip in Election 2008 to the Democratic Party. President George W. Bush's job-approval ratings were in the 20s% and 30s% his final two years. Voting patterns were historically against the incumbent White House party. And Bush was narrowly re-elected in 2004 by 2.46% (nationally). When you consider the financial meltdown in September 2008, the only question is: Would John Edwards have won traditional routes with the electoral map (along with his North Carolina, West Virginia, Arkansas, Tennessee, Kentucky, and Georgia to go with Virginia and Florida)?
