If that should happen...then Habs will be forced to probably trade Domi, or Danault or Tatar or Petry, etc.If Duchene gets 10mil and Domi gets 70-80pts he will ask for 10mil too.
But in the end Domi will probably ask for 10mil regardless if he has a big year like Duchene did. So you will need to retain him plus find the player you are still missing because you didn't get Duchene. Who will end up costing 10mil probably.
And get a pretty good return.
In a cap world, there's no such thing as attachment to players, sometimes you're forced to move players you'd rather not move but if you play your cards right, you can maximise the return and create value elsewhere.
It's a puzzle, the best GM's make it work. Any player you acquire has a domino effect on the rest of your roster.
This is of course all under the premise that Duchene signs for 9-10M