Let me make this argument in a different way.
For the duration of the movie, he’s doing rehearsal, and they all go terribly. By all measures, he doesn’t know what he’s doing on a stage, and this shows it.
His daughter says, “Are you going to be able to do it?”
He decides, “No. I’m not going to be able to do it.” This is during the time that he gets drunk and embodies Birdman by flying through NYC. Birdman is no longer someone outside of him – he has cawed, and accepted that he is Birdman.
Now he no longer has to even worry about doing a Broadway worthy performance – he cannot, he’s decided, so he’s going to give them Birdman. Blood and Guts. He has changed.
Does that sound right? Because I’ve got to go figure out if I believe it!