By that logic all the revenue that the Harlem Globe Trotters generate is proof that a substantial number of people believe they will get to see the Washington Generals be victorious.
What a terrible analogy. People pay to watch the Globetrotters for the same reason they pay to watch Star Wars: entertainment.
You claim that
no one thought McGregor had any chance but you have provided no evidence to back up your claim. In point of fact, of Mayweather's last 10 fights, only two (Pacquiao and Canelo) have been less of an underdog than McGregor was. Even Shane Mosley (-400) was more of an underdog than McGregor (-350)... and Mayweather's last opponent, Andre Berto, was nearly ten times more of an underdog (-3000) than McGregor was. So I'm not sure why you see this fight as so unique. Mayweather outclasses everyone he fights.