Not only that... but he's still only 25 years old.
The reality is, if the Leafs put Matthews on the block, there will probably be a list of upwards of 20 teams making calls... simply because it will probably be the only chance to get a player of his ilk.
From a Leaf standpoint, given that they'd be trying to turn around and continue being contenders, they have to be somewhat specific on the package. It needs to include somebody who is around Matthews age, or younger, that can comfortably slot in as a top 6 C on a contender.
The package also needs to include something to satisfy some of the other areas of the roster -- whether that's a big defenceman, a goalie... certainly can be up for debate.
If you're looking at Arizona, I'd imagine it's a package of Barrett Hayton, Lawson Crouse , Conor Geekie and a pick or two.
Before everyone jumps in and says "they'll just wait for him to become UFA"... it's not that simple. The Coyotes need a rink, and they kind of need the deal done this summer. Getting Auston Matthews, likely gets them their rink.
There is also the risk / reality, that if AZ doesn't step up to the plate on a trade, that the Leafs could go and trade him to LA, or some other reasonably desirable destination, and that team turns around and gives him something to the tune of $14 x 8.