You've actually touched on my 3 main concerns which would dictate how much salary they should get:
On which line they'll play: it's a match-up thing - each of the lines are supposed to match up with one of the other team's specific lines, and I get really, really nervous any time they start on the defensive zone. I know they like to say that young guys need to do better, but in all honesty, their effectiveness has been putrid on so many occasions: they cough up the puck with increasing regularity; their passes are going nowhere; their shots are getting weaker. In fact, their lack of effectiveness on the Marchand goal hasn't been an anomaly.
Right now, you're already hoping that they will be able to get 12 to 15 minutes of good Sedins. But based on the current time (just think about how many points they've scored in 2017 and the increasingly smaller number of good scoring chances they've generated even though they've had a lot of offensive zone starts), I strongly doubt that you're getting even 7 minutes of good Sedins. That number's going to decline even more. If you extend them even 3 years after next season, what sort of production do you think you'll get on years 2 and 3?
It's one thing if you're spending important cap space on one popular but declining vet. In this case, you'll be spending it on two players. Split them up and hope for the best? Will they take an extension of 1 year? Perhaps extend one player and trade the other (sacrilege!)?
Again, I have been a big Sedins fan pretty much since they were drafted so much so that I actually spent time going through the NHL's Top 100 list trying to see which ones of those players should have gotten kicked out of the list so that Henrik could make it.