You're mixing one thing with the other. No one's denying the need for Arnold's presence, but there were enough areas of the film outside of that that could've been more inclusive of other BBs.
The problem that you're apparently unwilling to accept is that greater accuracy in the book would've interfered with the star vehicle the movie was meant to be. There was really no need for segments on Columbu or Katz when other BBs or training could've been shown that would've better reflected the book.
What part of "docu-drama" didn't you understand?
Furthermore,
Pumping Iron would have been filmed with or without Arnold. Without him, it wouldn't have done as well. Remember that he wanted to be in the movie, but he didn't want to compete. George Butler didn't want it like that. If Arnold didn't compete, Butler wasn't going to put him in the film.
According to Arnold, the investors thought a lot of the footage was boring. That's a reason why they set up scenes to make things interesting, such as Ken Waller's "plot" to fluster Mike Katz by hiding his T-shirt. As you may know, that scene (with Waller, Robby Robinson, and another guy whose name I can't remember, throwing the football around) was shot after the Universe.
The movie does a nice transition: From Arnold guest-posing at a prison, to pics of him in the magazines, to an amateur (looking up to Arnold) getting tips from the Oak, to that amateur competing in a local show, to Mike Katz guest-posing at that show, to Katz's background (starting with him playing with his kids), to his commentary on his competition at the Universe and seeing Waller as his chief competition, to the football scene with Waller and crew, listening to Waller criticize Katz's physique he plans to "mess his (Katz's) mind up a little bit".
With Franco Columbu, the only interesting thing the filmmakers could do was have him move that car in his old home country of Sardinia.
Most of what I've just mentioned takes a good chunk of time from the film, WITHOUT Schwarzenegger involved at all. The films showcase two contests: The Universe and the Olympia. They wanted to make each contest appear to be a two-man show: The Mr. Universe, being between Katz and Waller; and the Mr. Olympia, between Ferrigno and Schwarzenegger.
That helped make the film more intriguing, setting up the "mind games", supposedly pulled off by Waller and Schwarzenegger on Katz and Ferrigno, respectively.
