Top 5-6 pick is not a "lesser" pick. It's likely generate a top 2 defenseman.
The Ducks already have their own pick in that range. Not that another pick isn't nice, but even if they pick two blue chip dmen, and as you presume they get two top pairing guys out of it, that means guys like Mintyukov and Zellweger are now topping out as 2nd or 3rd pairing guys? Their other good D prospects are topping out as 3rd pairing players, or buried in the minors?
At some point there's so many guys on D that they would have to trade one for a skilled forward. You know, like Zegras. So why do that in the first place?
The Ducks aren't a bad team that has a barren system in desperate need of restocking, they have one of the best pipelines, have 7 picks in the first 70-80 this year, and already have numerous young blue chippers already in the NHL. In all likelihood next year's D looks to have 4 first or second year guys playing regularly. They project to have Carlsson, Zegras, McTavish, Terry, and Gauthier in scoring roles. They're likely adding another major piece this draft. You really think they should still be in teardown mode of accumulating picks?