Auston Matthews had a very close relationship with Kyle Dubas. The two are at the top of their field and they both wanted to bring a cup to Toronto. However, with Dubas being fired, has led to uncertainty regarding Auston Matthews' future in Toronto.
Obviously, the Leafs need to find a new GM but also a new GM that will be able to sign Matthews. If Matthews were to resign, the Leafs will contend for however long that contract is signed for.
If Matthews were to walk, however, the Leafs would almost immediately go into a rebuild and many firings would take place and lots of people will be upset in Toronto.