NMIXX's blind sales were less about Big 3 privilege and more about recognition of JYP's brand with girl groups. People who think the company have a good track record aren't going to blink at paying $20 for something like that. It's no different than people who love eating at Popeyes or Burger King being willing to try a new menu option or even people rushing to preorder Elden Ring based on the studio's previous work.
Brand recognition is one of big3 privilege.
You are right with the burgers. Lol
But now the burger tastes bad. Still they will sell/buy the same amount or more next time. While most would never try to taste a burger from a small shop even if half the population say it tastes good.
Its like McDonald's burger. Low quality mass produced because the majority of the audience will eat whatever they get served anyway. Those who want quality dig deeper to find a shop that suit his/her taste.