Why do people doubt Prince Fielder as a possible homerun king?

Five hundred home runs is very achievable for Prince. He's going to have power, and even if he gets a few injuries, the DL will salvage his career. Just look what it did for Frank Thomas. The bigger questions will be his ability to hit for average, hit clutch, and carry his team the way a true power hitter should. Look at Manny, the most complete hitter in baseball. He carried the Red Sox singlehandedly to two World Series, and propelled the Dodgers into the playoffs. Now that is true domination of the sport.