Part of the reason I hope to do that is because I kinda wrote that I would consider a Hall for Larsson swap... About two hours before the trade broke. Here are the reasons:
REASON NUMBER ONE
If you're anything like me, by now you've read comparisons between Adam Larsson and the best right-shooting defenders in the league. Sadly, Larsson does not fit into that very exclusive group - at least not yet. But the point is not that Larsson may get better, it's that those top pairing, right shooting defensemen don't get traded. The Oilers weren't getting one. No, not even PK Subban, who was traded for another player like him, which the Oilers lack.
REASON NUMBER TWO
Adam Larsson is in a sweet spot of his career that could pay off very big for the Oilers. He's young enough that he's not untouchable in a trade, but he's old enough that we know he is capable. He's on a very team-friendly contract for a long period of time, and many of those years should be his prime. They also happen to coincide with McDavid's prime years, and Larsson's contract may help them absorb McDavid's raise better than the big ticket of an established defender. There aren't many players in the league that fit that description; even fewer could be acquired in a trade.
REASON NUMBER THREE
The player that you would consider to be fair value for Hall wasn't getting traded for Hall. There was probably never going to be a Hall for Ekman-Larsson swap, or Hall for Pietrangelo, or Hall for Subban. Those players would leave gaping holes that Hall simply does not fill. There was no getting the Taylor Hall of defense in exchange for Taylor Hall. Also, the Oilers have had a tendency during the last decade to hold on to players when they have value, and trade them when their value is at its lowest (think Hemsky, Perron, Gagner, Petry, Dubnyk, etc). Does that mean you have to trade Taylor Hall? Not necessarily, but Jordan Eberle and Ryan Nugent-Hopkins don't have the kind of cache around the league that they once did. Even in Edmonton the bloom has come off the rose a bit with Eberle, so imagine the rest of the league's perspective. They probably could have traded him for a lot when he scored 30 goals and 70+ points, but that ship has sailed.
REASON NUMBER FOUR
The Oilers shifted their strength from left wing to right defence. From a purely positional standpoint, that's an improvement. Larsson doesn't give you on defense what Hall does from the wing, but this team could barely be called a team at all because of the lack of balance on the roster. At some point, one of the big names had to be shifted for some help on D. The optimal time may have passed already but it still needed to be done. That's not Chiarelli's fault, per se, but he'll deal with the consequences.
The Oilers lost this trade, in a vacuum, no question. Hall > Larsson. However, there is the beginning of a team here. At least I think so, you may disagree.