That's overly simplistic I think. There are a lot of reasons that the Habs are now bad, you can write a novel about it. Bad trades, bad drafting, bad signings, bad coaching, bad development...the list goes on. I think the biggest reason they are bad is because they can't draft, though. I think that's the overwhelming issue here. Sure, it doesn't help that they traded Subban and Sergachev, but Drouin and Weber aren't bad players. They're getting borderline nothing from their prospect pool, and anyone they get is being hurt by their bad development of players.
Since 2008, the Habs have yielded only a handful of NHL regulars from the draft. They are Gallagher, Beaulieu (not on Montreal anymore), Galchenyuk, Hudon, Andrighetto (not on Montreal anymore), De La Rose, Lehkonen, Sergachev (not on Montreal anymore) and Mete. That's straight up awful drafting, especially when you consider guys like De La Rose, Hudon and Andrighetto aren't hard to draft and aren't special.