Reading this thread is clear that the players are divided when it comes to this, and think the solution could be, you can have your challenge and reward, to make things interesting for you I supposed, and at the same time the content would have to be available in sandbox. I will give you my example, I have been playing the game since the beta came out, and not even once have I click on anything other that sandbox. Since my only interest in the game is its amazing construction aspect and being able to create my own zoo design. So having to play a challenge that I'm not interested in to unlock a item or animal that I would really like to have for a sandbox mode built, would be extremely inconvenient and unnecessary troubling, at the same time I would not want anyone else to deprive of getting a reward after completing a challenge, so let us have both.