What does London prepare the readers for John Thornton's death? Could the story have ended without John Thornton's death?

I think John Thornton represents the positive almost unbreakable bond between dog and man. Thornton has to die for Buck to move on to finally answer the call to the wild. Buck would not voluntarily break his loyalty to Thornton but his death finally gives Buck the opportunity to answer his call to the wild. Once Buck fulfills his debt to Thornton, he exacts vengeance on the Yeehats, Buck takes his place of alpha-dog in the wild finally free from the bonds of man.