what did Warren Magnuson do wrong to lose?

[Ogre Mage]

1980 was a terrible political environment for Democrats, like 1994 and 2010.  By 1980, Magnuson was in his mid 70s and his health had begun to fail.  Gorton was a first-tier challenger -- a three-term Attorney General, intellectual, vigorous and a relatively youthful 52.  He portrayed Magnuson as past his prime and ran on the slogan "It's time for a change."  I think you are correct that Carter's early concession cost Magnuson votes but given the margin of victory he surely would have lost anyway.  You could also put it another way and say that Gorton benefited from Reagan's coattails.  Also, Washington was much more of a swing state in 1980.

The comparison with Patty Murray is interesting as I think she shares several traits with Magnuson (whose old desk sits in her senate office).  Most notably, both shared a talent for steering federal dollars to Washington State.  Some thought Murray might go down in 2010 the way Magnuson did in 1980 (though I was not one of them).  By 2010, Washington had become much more of a Democratic state.  The still vigorous 60 year-old Murray showed no signs of "slowing down" and two-time loser Rossi was a weaker challenger than Gorton.