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Disagree. Why does it have to be either extreme scenario? Why can't it be something in the middle? The more realistic scenario is that they get their name out there, gain a number of new fans, and call it a win.
Expecting either extreme scenario is unrealistic. Only K-pop fans put so much weight into a Coachella performance. No Coachella performance has made an act significantly more popular overnight.
There's this misconception that Coachella gave Blackpink their "big break" in the West. Not true. Blackpink already had momentum right before they did Coachella. Coachella was just one small chapter in their journey that year, but they would have done it with or without Coachella 2019. If anything, it was "DDU-DU DDU-DU" in 2018 that gave them their big break in the West.