I hesitate to post but given the discussion over passengers' awareness of an issue apparently until close to the end;
Is it likely that due to the angle of descent, it would perhaps to a casual observer only seem that the cockpit door had malfunctioned and not that someone had sought to take control of the plane with intent to harm?
Even seeing crew trying to break the door might not automatically lead to the supposition of deliberate locking out, especially if the plane was still descending in a roughly horizontal manner. There have been prior door malfunction events quite recently.
It may only have been when the proximity to terrain became evident through the windows that people realised their lives were in danger.
Hope this is not unwelcome speculation, it just seems to make sense to me given the facts we know.