Another reason they are set up so well longer term is that (presuming Gurley's 5th year option is picked up and they kick in Donald's extension) their biggest free agents of 2019 are Saffold, Jamon Brown and Havenstein. Literally no other key players hitting free agency next year. Of course Whitworth and Sullivan will be a year older and personally I believe it's highly unlikely Whitworth comes back in 2019.
So ideally they go through 2018 with the same starting OL while adding better depth this spring to get a head start and then dive deep into the next generation of starting OL next year.