This is great news for Leafs fans, as locking up Auston Matthews has all sorts of benefits.
The main and obvious one is that Auston Matthews would remain a Leaf. A best case scenario is for the Leafs to lock him into the max eight year term, however due to the insane amounts of money he would be forgoing to do that, I would expect Matthews to sign for five years.
It makes him super rich, cuts his risk down, but still allows him to sign a contract where most of the years left would be before an age related decline.