Kings trading Doughty would be like the Penguins trading Malkin with 1 year on his deal. What would you trade for him? If you wouldn't be happy accepting something like Toffoli + first for Malkin (assuming he only had 1 year on his deal) then the Kings obviously wouldn't be happy accepting Guentzel + first.
Also, I know Guentzel is possibly better than Toffoli but I hope you understand what I'm saying. A 50-60 point forward and a late first isn't enough for Doughty. What I'd ask for if I was the Kings? A top 6 NHL forward under the age of 25 and also under team control for a least 3 seasons + top prospect + 1st round pick + 2nd round pick and a conditional 2nd round pick (becomes a 1st if he re-signs or you win the cup in 2019). Of course things can change depending on who the players are and/or what # overall the picks are. So for the Penguins, at a very minimum I'd accept Guentzel + Sprong + 1st + 2nd. And to be honest I'm not all that thrilled with getting Guentzel since he's a smaller forward. Really good player and I really like his game but he seems to get hit hard a lot and the West has been known to be the more physical, grinding of the two conferences. Also have no clue about Sprong's game but I read your blog from time to time and you guys seem to love him.
It's also pretty likely that Doughty re-signs with the Kings so the Penguins or any other team would have to blow away the Kings with their offer. Kings are not looking to trade him at all.