If Gustafsson takes the rematch and loses again, he drops pretty low in rankings and sort of gets forgotten. What's the point in staying in light heavy weight if you can't beat the champ after 2 shots at him?
If he doesn't get the rematch, he rides the crowd love and stays high in the rankings. People will still talk how he got robbed until he gets his shot again. Ride that buzz for as long as you can. Fight a top contender and then take your shot again.