The other factor is that a projector is the first part of allowing others to view films, and getting the light source nailed down could open the doors to making new prints of those films - a different path to archiving.
You're missing the point. In this case, the point is literally preserving the experience of viewing actual film: IE, preserving the original viewing experience that the film was shown in.
It's kind of the same think as listening to a vinyl record from the 1960s, even though the digitally remastered 24/96khz flac is technically more accurate to what actually happened in the studio. IE, if I want to know how my parents enjoyed the Beatles, I pull out a vinyl record, even though the various digital remasters, including the recent 24/96 versions, are "more accurate" to what's on the master tapes.
I think the question was more about the capture of fine detail. A scanner will digitize much more image detail than any capture of the projector output. Although, reading the article it seems an emphasis was placed on color accuracy. I'm not sure if a scanner is necessarily as good at that.