Eh, I'm ultra-critical of Marner's leadership but even I think he's going to keep that A. While he can't hack it in the playoffs, he's still one of our team's key leaders 82 games a year and a central figure in the dressing room. I think taking the C away from JT is the big step so that Matthews can now fill that role, I don't think they need to force Marner out of a position that the players likely see him in.
The big difference is that JT is a weak leader and it's critically important that your Captain be a strong leader (and there's many different successful leadership styles), Alternate captains are subordinate leaders and you have a lot more flexibility for strengths and weaknesses there. We needed to fire JT from the Captain's job, while we need Marner to embrace his job as a team leader. I'd argue that his playoff struggles are largely due to a focus on himself, rather than leading his teammates by example.
I still think we should trade Marner, but if he's here, he's deserving of the A. The goal is to either cut bait on the player or to rehabilitate him. Humiliation is not productive.