This Movie Isn't About Audience Guesswork
Too many questions left unanswered. Your World Trade Center example just doesn't fit. There is no comparison. The events, as is typical of American fast and simple, are compressed into a few minutes whereas the long, drawn out days and months, and years of The Pianst just beg for explanation. World Trade Center is visceral, action-oriented, thrilling, popping action of the terror and horror of war whereas in the case of The Diary of Ann Frank as well as The Pianst we are looking at the lengthy period of the traumatic exposure to Holocaust. The audience isn't interested in knowing what they themselves think, the audience is interested in the character. How do events impact the character not us. What was the character thinking? What was happening while he was sick during the lengthy time in bed. What thoughts were going through his mind? How did he think? What were his emotions? What was his reasoning and how did they deteriorate. The surface features as just that superficial without the supremely unique feature of human thought and feelings. Each person is unique. If the picture were about animals, a voice over wouldn' t be necessary, because the dog or cat emotions and surface features are all that really exist and count. But even the focus is on a singular individual over the length of this movie oer a period of years, the inner most spirit of the human mind is what makes this movie truly special and without it, The Pianst becomes more an empty shell to be filled in with everyman's experience - not too inspiring project then.
Film is a multi-media experience
Just because film originally was a singular visual event when it was first invented in with 1890s and then sound was added soon after and became common place by the 1920s, 1930s...doesn't mean that motion pictures must restrict themselves to the visual experience...as the many Disneyland rides that rely more on physical, visceral experiences that provide an emotional high and active excitement, true serious drama shouldn't necessarily ignore the opportunity of bringing the greatness of the written text, the book, the novel to the screen. The opportunity to combine both the intellectual depth of the word and sound with the eye-popping, collision of color and sight should not be overlooked.
To simply expect the audience to become dumbed down by omitting the more important meaning and mental thoughts is to deny the quintesential element of motion picture's potential. Dr. Zhivago is probably the classic example where the omission of a voice-over was at its best. Comparisons of The Pianst again to a movie like Saving Private Ryan, again ignores the context in which the beginning action takes place in real time over the brief (long for those experiencing) with bullets whistling by, one's life on the life (there was no time for thought). Survival by instinct and just plain human emotion is just what Saving Private Ryan was about but not The Pianst. There must of have been thousands of thoughts that the audience never had an opportunity to experience to make the substantive and sophisticated connection of the Holocaust real in the context of human thought, not just emotion, not just experience, but human mental thinking.
What was Spelmann thinking while sick. Was he a selfish egoist more concerned about himself and his art? Or was he despondent over the deaths of his family? Was he delusional? What did he think of his neighbors? Was he scared or just plain bored? Did he think about music while in bed? What kept him alive his thoughts, will-power, his dreams, or just plain luck. It seemed that he lived life passively, letting luck and other people take him along, surviving not through any real unique character but being in the right place at the right time having the right talent with the right people. It's hard to believe that this particular movie was anything really significant, meaningful...except a brief excursion into an experential horror of war and "the accidental tourist" of everyman caught up in it.