First off, it's a full team effort to be better defensively, not just the D-corps.
KA was gifted Dahlin and Samuelsson when he took the job, and gifted Power as a #1 pick. Other than that he's drafted Novikov, and Lindgren and Vsevolod last draft in the mid rounds. No not much there but you could say he didn't need to have urgency at that position with what he had coming in. Coming into this season it seemed as though a top-four was in place with Jokiharju as a bit of a question mark. We still needed a defensive RHD and he went with a bargain signing with Lyubushkin. I can't blame him for what he's done, but I think now it's clear that a better bottom pair is in order. This is part of the "let's see what we have" process for this season.
I'd like to think KA will be more aggressive in shoring that up in the summer. Maybe if Ryan Johnson signs the need is addressed to a degree. I'd like to see a FA signing like Ryan Graves or a trade with LA for Matt Roy, because even though Bryson has played better lately I just don't see him as being good in a playoff series, and Johnson may need time and may not even work out. Some of these guys look better at times in the regular season but we will need players that can be impacts in the playoffs. After new contracts for Cozens and Dahlin, and maybe Power and Mittelstadt are done...KA can see what cap space he has to work with and go after a proven vet or two.
If none of this pans out in the summer then I will be a big griper about KA and the D-corps.