You should take a look around the league and you'll see how often things like this happen.
Rielly is signed. Barrie will almost definitely be signed, and if not, Muzzin. Dermott will be re-signed as an RFA and should be fairly cheap and ready to step into a 2nd pairing role. Their 2 best prospects are both D and developing amazingly. Everybody was screaming about our downfall this offseason, and how we were losing half of our D, and losing our depth because cap. And now we're entering next season with a much better and more balanced team. Dubas has a plan, and he is executing it perfectly despite a bunch of obstacles.
Even in a worst case scenario where they have to trade somebody, they will still have 3 franchise players, and 2 of the best Cs in the game. And a #1D. And a #1 goalie. Leafs have been a playoff team despite everybody being a rookie. Leafs have been a contender despite not having Tavares. Leafs have been a contender despite essentially not having Nylander. Leafs have been a contender with 38-year old Hainsey as their best RHD.
The forward core will be locked in for 5 years +, and a lot of the secondary scoring is even locked in for a few.
The only way the Leafs fall substantially back is if they lose Rielly or Andersen, and I see the chance of either of those happening as essentially 0%.