Could have, would have, should have.
Honestly, Taylor Hall has had one good season since becoming an NHL regular and even then, that one season has to have an asterisk on it because he was a -14. He's only been a plus player once in his career. I know people will say "well, he played in Edmonton", but that can only bear so much weight.
Fact is, if he wanted to be one of the 'pillars' to be built upon, his defensive game would have improved, and it hasn't. As well, you have to ask yourself - if Chiarelli has a list of untouchables, and if Hall wasn't on the list, why would he be the first to go? You'd have to figure that Eberle and RNH could have brought back a decent package, so why did they stay and why was Hall moved? Maybe there's more to moving Hall than just 'that was the asking price.' Maybe Hall was one of those guys MacLellan couldn't get through to and maybe he had to go.
As for Larsson, I think he'll rebound just nice. Don't forget that Peter DeBoer nearly ruined him. It wasn't until DeBoer was fired and Scott Stevens got to work with Larsson that his game began to turn around. Then, John Hynes took over and got him back on track. This is a big, smooth skating defenseman that is going to be the shut down guy in Edmonton. He's not going to be asked to run the power play and a guy like Darnell Nurse might have that room to roam a little bit because of Larsson.
This is a good deal for both teams.