With Mitchell for Jansen, it’s a hard pill to swallow but that’s what it would take for a catcher who hits like him. That, plus the OF surplus and the blue jay need for a lefty batter and to get Springer out of CF makes it make sense for both sides. If it was an easy “yes” trade, it’s probably not even.
With regards to Diaz, he’s literally not just an average guy. Last year he had an OBP over 400 and OPS+ of 140. Those are both far and away better than any Brewer. And with his ugly D at 3B he’s make for a much better 1B/DH/occasional 3B than any current options. His hitting is what the team needs, and he’s controlllable so it would take a talent like Weimer to get him