So the entire point of the Fellowship was to take The One Ring back to 
Mount Doom, the only place where it could be destroyed.
My question is this: Why couldn't Sauron just, you know, sit around inside 
Mount Doom? Why park yourself in a tower 100 miles away? If he'd waited around at the entrance to 
Mount Doom there'd have been no way for anybody to ever destroy it.
Thus, Lord of the Rings has been ruined!