The ultimate story of 2001, for me, is that Kubrick was one of the first to really use audience screenings to effect the final version of a film.
As I have read the tale Kubrick put behavioral psychiatrists into the audience (and in observation ports around the theater) and those observers would note whenever the audience got bored or exhibited any loss of interest.
Kubrick then cut that portion of the film, no matter what was going on or how important it was to understanding the plot.
True or not that always explained a lot about 2001 for me.