The way I see the original Duchene trade is that Karlsson had 1.5 years left on his deal, so that was the Senators window. With Melnyk's budget, it would have been very difficult to re-up Karlsson, Turris, Stone, etc and compete under 68 million in player salary with Phaneuf and Ryan on the books as well.
I think there's a chance Dorion looked at it like he expected the Sens to compete for a playoff spot, and if there were competing for a playoff spot at the deadline, they weren't liquidating Turris who was a UFA. At the same time, they did not want to extend Turris. He was a sunk cost they expected they wouldn't be in a position to trade at the deadline. It was also pretty well hinted at by some people that he believed the addition of a legitimate center in Duchene would transform the team into a contender.
Meaning, he probably thought he was giving up a mid 1st, Bowers, and a 3rd to upgrade Turris to Duchene. (Because again, Turris was a sunk cost). Then, if Karlsson didn't re-sign and the team had to rebuild the next year, he could have gotten back the equivalent of that mid 1st, Bowers, and a 3rd by trading Duchene away at the 2019 deadline. So he would have upgraded from Turris to Duchene for effectively almost nothing asset wise. (Since again, Dorion probably didn't expect to be in a position to sell at the 2018 deadline with Turris).
We all know that the losing streak started, "Gretzky was traded", and everything else is history. I think that might have been the logic with the trade at the time though, Turris was a sunk cost as a UFA they weren't going to trade if they were in line for the playoffs, and Bowers/mid 1st/3rd could be recouped next year if needed by flipping Duchene at the 2019 deadline.