The main reason you extend early is to lock in this year's prices before inflation.
This is generally sound thinking and might be worth the risk, however, when it comes to the RB market, chances are that we will see deflation instead of inflation.
I believe with the releasing of Gurley, the trade of DJ, the Chargers coming out looking smart, etc, the RB market has hit a peak and is about to go down.
When McCaffrey's contract would have been up to extend or re-sign in a couple of years I am guessing the contracts will have come down on the RB position a bit. The Zeke and Gurley contracts will long be forgotten, and he most likely could have been kept for at worst, the same they just paid him. So in other words, there is no reason to take the risk right now at all.