Why, in all the arguments about digital vs. real film, does the
"elephant in the living room" get left out of the discussions so
frequently? I'm referring, of course, to the simple fact that as of the
present moment, long-term survivability of digital images is something
no one can predict or guarantee.

Here are just a few considerations:

1. Software obsolescence: will the next generation(s) of programs be
backward compatible?.
2. Technological hardware development and obsolescence: will the next
machines render images inaccessible?
3. Media for storage; how many hard drives must one have? And even
they have a mechanical shelf-life. No CD's known to this point are
reliable long-term.
4. Digital print longevity: getting better, but nowhere near B&W
silver images.

I have no quarrel with commercial photogs who use digital; makes perfect
sense. Theirs is an ephemeral world anyway. However, I do worry what
images historians and cultural anthropologists 200-300 years from now
will have to work with if film loses to digital. Photographs, among
other things, are records. We can study ancient Egypt because they
carved their hieroglyphics in stone. We can appreciate the beauty of
medieval manuscripts copied on paper with pigment inks. How will future
generations study us?