This is basically the opposite of the scientific method. This is we've tried nothing and it hasn't worked! Changing the 3rd line center that Matthews doesn't even play with isn't a valid method for determining he's the problem.
To figure out the problem, you slowly remove core players and find out if things change. The thing about Matthews is his game has no weakness aside from health. At his best, he's a strong goalscorer, he's a strong faceoff man, he's strong defensively and he can be physical and play in a physical series.
The problem is he's been surrounded by players with clear weaknesses and I think eventually he's like why do I have to be the only one who goes in on the forecheck, or hits, or cares defensively, etc. You put Matthews on Florida and he would shine. You need to build a team around him that plays the way you want him to play.
What is clear right now is this core is the problem. That doesn't mean we got to get rid of every player in the core. How about we keep the 69 goal scoring Selke-finalist centre that we'll be dreaming about when we trade him?
We can start to explore the changes of the team when we hopefully walk away from the softest player of this core, Marner, this summer.