My hunch has been each team has about the same number of dedicated fans and it's not that greatn of a number. Maybe 20% of the fans that will engage with one of the teams. After that it's, take it or leave it maybe go to a game or do something else, convenience, standings, star player in town, but they would go to either one. It's all fighting for what some would call the band wagon jumpers.
I did most of my baseball entertaining at Sox games. Dinner and a game without the buyer leaving their office early was easier than a game and dinner in most cases.