Cook is a really good player. If I was 12 years old, I would be upset if the Bills let him walk. But despite the impact Saquan Barkely and Derrick Henry have had on their teams, its not clear either of them are ending up Super Bowl champs.
If the Bills win the Super Bowl this year, I happily run back every player on the team including Cook for next year. Otherwise, it is simply a question of what trade offs I need to make to keep Cook vs. some other players.