That's a fair assessment, but the Habs lack the cap space to sign him. Winnipeg isn't taking one of Montreal's garbage contracts back like Gallagher or Armia (who they dealt to Habs) or Byron or Drouin or Hoffman ect. The Habs will have to trade a roster player back to the Jets to make the necessary cap space to sign Dubois. To sign Dubois they're probably going to have to make $7M or so of cap space. They have just under $250K of space according to capfriendly.com. If Price goes on LTIR it will help, but that hasn't happened yet and they still need to sign Dach. I'm guessing someone like Anderson and/or Dvorak will have to go the other way.
You also metioned that Winnipeg should expect a rental type return, which for a guy like Dubois would be a 1st rounder, a good prospect, and possibly another pick/prospect/player. I'd be very surprised if a 1st, high end prospect, and good/solid roster player isn't included in a deal with Montreal. I also think that other teams would be willing to risk a rental return for him even if he only signs a two year deal with them. If the team that lands him offers a big money 8 year deal, I'd be surprised if he walked away to sign in Montreal. Especially if the Habs aren't in a position to match that hypothetical deal.
I'm not against acquiring Dubois. I just don't believe it's going to be a bargain like some are suggesting.