However, what you're stating is not true.
I totally agree that the first week was a bust, but you have to take into consideration that the games took place in the afternoon, during most people's work and study hours, while the most important holiday of the year was happening (when people travel the most). Also, Riot sold the tickets without announcing the teams playing at the day beforehand, meaning a lot of people bought tickets to watch teams that they don't support.
That being said, last couple weeks the venue was kinda full, specially during the weekends. I was there on Feb 25th (TS x Navi / Lev x Vitality) and I can comfortably say that the arena was full. Same goes for today (~80% full), even though it was a normal working day in Brazil.
I can affirm to you that the finals would be full regardless of which team was playing.