A proven commodity who DIDN'T WANT TO PLAY HERE. As Butchy pointed out Dorion tried to gauge his interest in discussing an extension in Ottawa and the Hawks declined to let him speak to DeBrincat's agent, which is all he should have needed to know.
You can't overlook that fact considering he was 2 years away from attaining UFA status and had a 9M QO in 23/24.
Dach was unproven but was 20 years old and was a recent 3rd overall pick that obviously had a lot of talent and upside. If the scouting staff didn't like the names available at 7 and Dorion was intent on trading the pick to speed up the rebuild, trading it for Dach would have been a far smarter way to do it as a Turris-esque buy-low move adding a talented former 3rd overall pick that was struggling.
He traded for DeBrincat because he thought it provided the best chance to push the team to the playoffs early, long-term success be damned. He didn't care about the eventual implications, as he wanted to be able to say he rebuilt the Sens into a playoff team to try to save his own job. It failed spectacularly and defending it is laughable at this point.