Richard Newman asks: "Mike, the discussions by Ctein, you, and others, of how long photographs will last, leads me to ask a few questions. There seems to be an assumption that all photographs should be saved forever, no matter what. So I ask: How long is 'forever'? Why all photographs? Will many of these trillions (or more) images be of any value to anyone? In some distant future, will anyone care?" [See the Comments section of the previous post to read Richard's entire comment.]
Mike replies: Obviously what matters most is that our archives remain accessible to us for as long as we might need them. For example, when I started thinking of having this spring's print sale myself, a picture that popped into my mind is a "lakescape" I took in the '80s on 4x5 color film. The work I did in color 4x5 was by far the most popular with other people of all of the photography I've done in my life; it accounted for most of my early sales (as well as most of my early compliments). But I can't find that negative. It's likely that it's in a box in the basement, and might well have been damaged by the humidity and mildew even if I am eventually able to find it. This is a basic failure of preservation.
Another case using myself as an example is that one of my more interesting pictures was taken on a job in the early '90s. It was "work for hire" and I never owned or possessed the negative. I think it's 75% likely that the archives of that employer from that time no longer exist, and that that negative is now gone. So what I'm left with is one small drugstore print that we used as proofs on that job, and one "fine print" on fiber-base paper that I made from the negative at work. Now that I'm thinking of putting together a book of my 60 or so best 35mm pictures—of which that picture would surely be one—you can imagine how important it is to me that the single print I possess was well made on durable materials.
The best case: set-and-forget
Most of everything will be lost over time, no matter what we do. What we're advocating for are a) materials that don't automatically self-destruct, so that the small percentage of them that happen adventitiously to survive can indeed survive; and b) that people think outside of their own immediate individual interests when it comes to the evidence of the things they've witnessed during their time on Earth. It's true that this is no more likely to be of interest to everybody in the future than our work right now is of interest to everybody; but it might be important to somebody, and it might even be important to history, society, or culture.
One problem is that "now" has no idea what "posterity" might value. Ctein used the example of childhood pictures of people who grew up to be President. If your grandson is a future world leader, you can imagine that your pictures of your children will be of great interest to historians—interest that you cannot right now possibly predict.