Counter point - All of those other young D were already in the organization when Anaheim went ahead and drafted Luneau.
They have an abundance of D prospects, but only Drysdale has shown to be anything at the NHL level. They need more than one of those guys to at least be an NHL player before we can start talking like there's some sort of surplus.
On the flip side, Anaheim's top (established) young players besides Drysdale, are all forwards - Terry, Zegras, McTavish. Plus in all likelihood, they are adding another high end young F with this years early draft pick. Not to mention the three second rounders they already own plus any other early picks added by the deadline, with which they can continue adding forwards.
In reality, these types of trades where recent draftees are swapped out, really don't happen. I'd be pretty certain that Anaheim was thrilled to land Luneau where they did, and are just as happy with his development this season. There's no real world motivation to move him, just to say the prospect pool looks more balanced on paper.