Dude, when Loyola has a deep tournament run, or Northwestern goes to a bowl game, suddenly, everybody's walking around my neighborhood wearing Loyola or NW swag. Yes, people get excited by a winner. Then they decide to become further invested, or not. If the team has sustained success, then, yeah, people consider themselves Rambler fans, or US women's basketball or whatever fans. When something becomes popular, people follow it.
Conversely, when a team starts losing, people stop going; stop watching. That's really not some profound discovery. I really don't understand the manufactured panic of "OMG!! nobody will be Sox fans!!" If they win, they'll come out of the woodwork.