While I agree it has become tougher in the modern era (William Jennings Bryan famously lost 3 presidential races -- but that was a long, long time ago), these pundits failed to draw a distinction between losing a primary -- and losing a general election.
Sure, the days of losing a general election, and then making a comeback, are probably long gone (the possible exception is Gore -- but he actually won the popular vote).
Losing a presidential primary, on the other hand, has less of a stigma attached to it, and may actually be seen as a badge of honor.
And it's certainly not heard of for primary losers to make a comeback -- even in the Democratic Party. Al Gore lost the 1988 primary, but won the 2000 nomination, after all (granted, that's not as recent as the McCain example of losing in 2000 before getting the nomination this year -- but it's not exactly ancient history, either.)
Hillary may well be back in 2012. The pundits who are cavalierly dismissing the prospect she may well rise again are comparing apples to oranges.
Losing a primary isn't a game ender; losing a general election is ...