And if you had Lidstrom for one year and didn't win the Cup that year..you didn't exactly gain a whole lot either. Teams operate on the premise of trying to win the Cup, so if you get a superstar you normally do so under the assumption you'll have a 5-10 year window to do so.
In this case, that window is 1 season guaranteed. Ottawa had no chance of winning the Cup in that year with or without EK so trading him was sensible. San Jose increased their chances this year, but only this year..that's less than what you'd get with a 'full' Karlsson. And that's why the trade is fair.