Who are you referring to as the big 4?
Doesn't appear obvious, , most would assume Dermott stays over Holl.
Dermott was a healthy scratch in the playoffs, alternating spots with Dubas' 1st draft pick as the GM that needs a spot in the lineup. Even if he doesn't go in the expansion draft, there's probably a decent chance he's traded post expansion draft to a team that lost a defenceman.
Holl has been a stable, permanent top 4 defenceman with size for $2m; throughout basically all of Sheldon Keefe's tenure.
Reality is -- short of trading an asset the Leafs really can't afford to lose in order to incentivize the Kraken, they're going to lose one of Dermott, Holl or Kerfoot. They can only protect one of them. While you might argue that Kerfoot is more important to the team than Holl, Holl is by far the most difficult one to replace with the cap space that would be freed up.
The Leafs don't need to "fear" losing Travis Dermott, Rasmus Sandin will step into his spot. The Leafs don't need to "fear" losing Alex Kerfoot, they can replace him likely for less than $3.5m. They cannot replace Justin Holl for $2m.
As for "fixing the future" -- I don't think it's all doom & gloom given that the Leafs do have Sandin, Liljegren, Robertson and Amirov.... but I do think they need to go out and find some assets with longer lead times and more universal marketability.
Graduate Sandin & Robertson. Trade Liljegren to try and recoup something like a 2nd round pick in 2022. Possibly cash out on Dermott for a pick as well; and then have the ability to use those picks at the deadline if it makes sense. Picks are a magic box that a GM can make his own, prospects a GM might like a guy, might not.