The interesting thing about this deal is how boom/bust it is. I don't see much middle ground here, it's either going to be a great contract or a horrendous contract. The deal is too long for Johnson to not suck during it, regardless of what you think about him now, he's not going to be an effective player at age 37.
The deal will be great if he gives 3+ years of top-4 D performances, regardless of whether he sucks at the end. The deal will suck if he does any less than that. I don't think there's a middle ground where he just plays to his salary level every single year, I see him falling way off near the end of this deal.