Netflix has some pretty decent tech-chops, so they probably took a good look at the number of discs subscribers were getting and comparing that to the amount of

Netflix has some pretty decent tech-chops, so they probably took a good look at the number of discs subscribers were getting and comparing that to the amount of on-demand used to figure out the percentage of subscribers who were close to 100% on-demand only and going from there. It isn't like it they just blindly said, "hey let's try this." It could also be the writer trying to add some drama to an otherwise straightforward part of the story.