You trade an asset to get a 'maybe' top 4 Dman. What does that do for this team? Pushes Moore down a slot? then weakens the forward depth to bottom 3rd in the league again, while pushing the defense up to 20th in the league, at best.
Now, sign Shattenkirk. Doesn't damage the very poor forward depth, adds a '3/4 Dman' in your words. Then if you trade Henrique for a Dman, at least the added Dman, on top of Shattenkirk, makes a major boost to the defense. Which can hopefully offset the lack of forward depth.
It has been said, that fixing the D, can't happen in one offseason. But in reality, it can.
Adding the best point producing Dman to the 22nd ranked PP makes a big difference too. It can also make 5 on 5 play easier too. Teams are likely to play more passively when taking a penalty can be so costly.