The Kings would have done it to go in a different direction. For example, if you trade Doughty after the 2019 season you are likely getting a somewhat similar return to Karlsson, a 1st and a pretty good prospect being the key pieces and you are likely picking high in the draft probably through this season, while letting younger players get a ton of ice time.
And despite Karlsson's unbelievable renaissance season this year, the trade certainly did make Ottawa better, they landed their future franchise player and their 2nd line center who are 21 and 23.
But in all reality, why in the world would they go in a different direction, 98 points, 2 points out of 2nd, made the playoffs, have a Hart Finalist...and a Norris caliber D in Doughty, Toffoli was young, Pearson, Kempe, Muzzin, Iafallo, Forbort, Quick had good numbers....etc...
As opposed to when Ottawa traded Karlsson, same year.....67 points, 2nd worse team in the league, Stone Hoffman, Duchene, (the whole thing with Karlsson and Hoffman etc) had one year left.....was going to leave presumably, didn't want to stay....
Vs Kopitar who was 2nd year into that 8 year contract, 6 more years, Hart Finalist etc......
I have yet to see anyone come up with any sports team that has done that....without financial pressure, or personnel issues (players wanting to leave)