It's true, but it's worth noting that while in modern times losing is generally a career-ender for candidates it is much less so for the operatives behind the scenes, some of whom spend their whole careers cycling from one losing campaign to another. The most striking example of this is Democratic consultant Bob Shrum, who was behind the curtain for a remarkable eight losing presidential campaigns. (See http://www.washingtonpost.com/wp-dyn/articles/A9895-2004Sep9... for a good profile of Shrum, written as he was on his way to his eighth defeat in 2004.)
To be fair, though, between the years of 1972 and 2004, the only two winning Democratic candidates were Jimmy Carter and Bill Clinton.
You could tell a similar story for any Democratic strategist with an overlapping career. Tad Devine worked for the failed campaigns of Mondale 84, Dukakis 88, Kerrey 92, Gore 00, Kerry 04, and Sanders 16. (He also worked in Ukraine on Viktor Yanukovych's winning 2010 Presidential campaign, alongside Paul Manafort. Small world.)
Also, if you work for a successful Presidential election campaign, you end up working in the White House, e.g. George Stephenapolous, Karl Rove, David Plouffe, David Axelrod, and Steve Bannon.
Candidates like Clinton falter because they suck. Kicking off your campaign Roosevelt Island and driving to Pennsylvania or wherever in a hotel shuttle van was the kickoff to a weak campaign.
It's very strange that losing a Presidential general election is seen as the end of a political career when in years past, candidates like Bryan, Stevenson, or Dewey were still seen as viable.
Also, your explanation of Dewey choosing a political stance that was widely held among voters but somewhat taboo among the political class and riding that stance to prominence shows that there's really nothing new or innovative in the wave of populism sweeping Western democracies in recent years.