I think that it's a mistake to believe that because some rare stuff from the 20s until the 60s - say - might have been quite interesting, that more from the past couple of decades is also worth retaining. The early years were great, too, because nothing like photography had happened, but once it became mass culture...
I think the 60s had some revolutionary visual tokens - fashions, for example - and with the snappers with the eye to snap it well, but apart from that obvious pop culture genre, what was there that matters? From the last couple of decades, it's an ever harder guess. Even the fashion interest has vanished into a mess of uniformity, a kind of absolute lack of character. And how do you photograph nothing and make that of interest? There is no fashion anymore. Well, not in the vox pop sense of the meaning of the word fashion. Perhaps there are the empty housing estates nobody bought; the dead factories etc. but those aren't things that form part of the general photographic gamut enjoyed by the masses, which I think is what we are on about preserving here. Anyway, I suppose all the newspaper files and agency shoe-boxes have that stuff well-covered. God, just think of the zillion picture libraries that exist... enough crap in there to float the next Titanic and also to sink it, if it gets cold enough.
Rob C