That's not necessary either. We should keep our core players (including Eriksson and Marchand) and fill the depth spots with all the prospects we have coming up. Hayes isn't really needed on this team. And I'd rather give someone something else to take Seidenberg off our hands than lose a good player.
You have forwards in the system ready to move up or close to it. Pastrnak on the roster now starting to make a difference again, Vatrano, Khokhlachev, Griffith in Providence, Heinen coming out of the NCAA next season. Maybe they won't be 3rd in goals scored without Eriksson, but even without any other additions they would be fine up front. The defensemen in the system of the caliber you need are still at the junior level and are years away, so if you're counting on the system to fill the holes on defense, they simply won't be filled.
They've already shown they won't give Eriksson the years he wants. They can't give him the money he wants under a declining salary cap and still afford to extend Marchand, Spooner, Pastrnak, and Krug in the next two seasons, AND add a player or two on defense. If it were so easy to lose Seidenberg's contract it would've happened already.
How do I improve their defense WITH cap space? With a free agent like Hamhuis if all else fails, or by trading for an RFA on a team tight to the cap like Lindholm, Vatanen, Trouba, or Dumba. The assets you could get today out of Eriksson would go a long way towards one of those deals. To accept that they can't improve on defense is to accept mediocrity regardless of who they have up front, it's that bad and no one they have today is going to make it better. My preference would be to trade Eriksson for one of these guys right now, but I don't believe that's happening as those types of players usually move in the offseason. Even with a trade you need that cap space to sign the player without the other team taking any on, if they could add salary the player wouldn't be available in the first place.
With where they are and the magnitude of their needs they absolutely can't afford to hold on to Eriksson without an extension and lose out on a 1st+ for the 2nd season in a row with a player in that situation.