It might be a personal preference here, but I have zero interest in Mittelstadt.
Zacha is doing great as our Krejci replacement. We need a 1C for Bergeron when he retires. I don't see Mittelstadt as that. Coyle at 3 C is a lock and 4C we have a very good Nosek, eventually Beecher. 3 of our 4 C positions are filled post-retirement. That's why people are mentioning Couzins so much. Boston's need is a franchise C, not a middle 6 guy.
Now, do you want to prove he is a future 1C on a playoff team?
Go right ahead. Would be an interesting conversation, no doubt.
But simply making him the centerpiece in a Deal where Boston gives up a lot is insufficient. You are going to have to sell him to the posters here.