Grate n Colorful Oz
Hutson Hawk
Petry has been our most consistent offensive player this season. He is a top 30 guy in the League for his position. If the Habs could somehow move out Alzner, and bring in Muzzin then sign Gardiner in the off-season we would have two perfect pairings with a strong offensive minded guy and a stay at home shutdown guy.
If Bergevin is smart (lol), the only non-boat anchor contract he would move is Tatar. Byron will end up being a steal so long as he doesn't lose his edge with the foot speed. Danault will end up being the highest paid 4C in the league; but we only move him when Poehling is ready. Suzuki can come in as a winger, and our team becomes very very scary.
Move Alzner and Tatar, sign Duchene/Gardiner and trade for Muzzin and we are a cup contender. Trading Petry, unless there is significant overpay (Nylander as an example) is just the wrong play.
You're banking on a lot of things to come to past. The likelyhood to manage this is not high.
The likelyhood we'll have a logjam on the right side in the next two years is much more likely.
Also, no one is saying to trade him just for the sake of it, but rather that there are compounding factors that makes a possible high return likely at the next TDL.