“let the school be as frei as it wants, and eventually it will cease to exist – let modern kids go to Landers or Ner Yisroel.”
The reason YU still exists is because there is a sizable crowd of people who will not send to Landers (kal vachomer Ner) because it’s too much to the right. Even if the Rabbeim left it will not cease to exist until that crowd either moves to the right or left and there will no longer be a market for YU to cater to.