Just my opinion but I never really liked the idea of keeping the winning team. I think it's more fun For all involved when it's completely random. Reason being is it keeps it absolutely completely fair and random. It's anybody's game by luck of the draw. By keeping in existing team that was a powerhouse last year some may feel that no matter the luck of the draw they are already behind the curve from the word go. Even professional sports go through a draft every year. I say put them all back in, shuffle them up and let the cards fall where they may. A "defend the title" and the rest being random changes the entire dynamics of the contest. It then becomes more of a king of the hill scenario. I don't know I just think it's more fun, exciting, and suspenseful for all involved when you know from the start that it's anyone's ball game.
At the end of the season you can still take that previously winning team members, add up their scores and see if they beat everybody again this year. If the did they're the all-stars.