What would happen if Wile E. Coyote caught the Road Runner? Or if Anakin could save his wife? Or if Littlefinger was able to win the affection of Catelyn Stark? All of these are impossible tasks, that, if they happen, would be an end all for the plot. Would Tom just continue living his life without Jerry, or would he feel an empty void and seek to find a replacement to continue his cat and mouse games?
Let's apply this to a none television plot. Take, for example, an explorer who spends their life trying to discover the secrets of the Mayan empire. Or a scientist that spends their life trying to find a cure for cancer. Or a guy who's trying to get out of the friend-zone with a girl? Or a person trying to get their dream job? Or even, a Donald Trump getting elected to the presidency? All of these are possible, but if and when they happen, it's like, "Where do we go from there?" Will an explorer be satisfied and never seek to explore again or will they feel that drive and desire to move on and explore the next mystery? And where would be the next step? Just some food for thought.