Johnson did get paid but as most contracts in the nfle go they tend to either get cut or have to take a paycut about the 3rd yr into it. This does help johnson but when he had his big yr he was in a man blocking system. He could be a little more dance and wait for the big play in his running. They now use a zone scheme which doesn't fit his style as well. He also will take a lot of punishment and speeds backs that get the ball as much as him tend to break down.
Remember with the contract that he is #1 a rb which means they can find a 7th rounder to replace johnson (just look there are less and less 1st and 2nd round rbs because it is a 2 man job). #2 look at how many contracts get played out that are longer than 3 yrs outside of the 1st contract and star qbs. #3 look at johnson's ego. He thinks he will rush for 2000 yds again...I can't see him getting within 600 yds of that if greene is taking carries from him. And that 600 is the closest. I think he is more like 800 yds from it. Remember greene was a 1000 yd rusher they just brought in to back johnson up. #4 greene can block where as johnson can't. So look for him to be a favorite to protect the young locker.
I would not be surprised if johnson is gone in 2 yrs and they go and get a speedy guy to split time with greene.