I've looked at catchers with high similarity scores to McCann. The risk of decline really kicks in around age 33 or 34 for players of that type. McCann is 29. A four year deal would cover ages 30-33. I don't see that as very risky.
Plus I think McCann will leave some money on the table to stay with the Braves. If not I'm ready to move on.
I'm not as optimistic as you. Mac is showing a lot of cracks in his armor and a move to the AL might be in his best interests.