I think that the issue you are running into is that these things also correspond to Engblom moving and Jarvis moving, to say nothing of Washington getting a better goaltender and stable coaching. Washington took two top defencemen and an elite defensive centre from Montreal - it was not just Langway, though he ended up being the best player in the trade. If you want to try and claim that people are going against contemporary opinion, and Langway's Norris wins were a debated topic, then look at this article from shortly after the trade:
https://vault.si.com/vault/1982/11/01/whos-on-top-in-the-swap
Engblom is implied to be the main piece, Langway and Engblom are called the best defensive pair in the NHL, and Jarvis is referred to as a peerless defensive centre. That's the view from the time of the trade, so they don't even know that Washington has upgraded its goaltending or the Stevens is going to be an impactful rookie on defence. It can be easy to put a big turnaround on one player, and that story has won Messier a landslide Hart trophy voting result and nearly won Shaquille O'Neal the 2005 MVP for example, but it's rarely that simple. I'm not sure that a team ever added to its defence in the way that Washington did heading into 1983 in NHL history, and it is not just adding Langway even though he was the biggest piece.