How many letters do you have from your great grandparents? For me, it's zero. For records to be preserved, someone has to care about preserving them. For digital data, that means copying it forward to newer media now and then.
I'm sorry, but there's a huge difference in preserving a letter and a file. I say this as someone who has worked in the field of preservation (mostly digital) for close to a decade.
Really simplified... For a letter to be preserved: put it in a box and take it with you when you move and hope that the house doesn't burn down. For a file to be preserved: keep backups to avoid data corruption and media deterioration. Also: control physical media obsolescence and format obsolescence. Most people don't have clue about these things. If our cultural heritage institutions don't get their hands on some of these files, most of them will likely be lost.
And no, it's not only about our great grandparents. It's also about the Turings and Einstens. Their day to day mail correspondence has taught us quite a lot. Turing might not have been a good example though... :)
The problems are simply different, not one is better than with the other. With paper and film, making backups is tedious and expensive and rarely happens. They get lost, mildewed, thrown away, looted, burned, flooded, blown up, eaten by insects, etc. Much of my family history got lost that way. Any piece of paper surviving 200 years is a miracle.
Paper archives have the unfortunate characteristic of concentrating treasures together. 9/11 apparently destroyed quite an archive of photos - we don't even know what all was there. The Vatican Library concentrates a huge collection of paper, and one little event could take it all away.
With digital, copies are cheap. I've been copying forward my old stuff for 40 years now (though I did lose my old IBM punchcard decks and paper tapes!), and it does get easier. With cheap terabyte drives, one drive will hold it all, and I can make copies to make it resistant to catastrophe.
The Vatican Library really needs to make it a priority to scan all those papers.
