I'm afraid I disagree. Given how many Jewish children were murdered in the war years, Anne Frank's diary is tragically highly unlikely to have been a one-off. Moreover, despite its literary merit, it only became valuable to the rest of the world once the world wanted to make a statement about Nazi atrocities, so it was hardly of value "from the start".

In the good old tradition of "what-iffing", if Germany had won the war, Anne Frank's diary would either never have been published or would have had to wait decades (even centuries) before prevailing opinion modified enough for it to become acceptable. On the flip side, we might have been arguing that Hitler's "Mein Kampf" was a one-off and of value from the start instead of the reality now where most see it as a historical oddity of interest only because of the extreme infamy of its author.

I'd agree it will be a rare occurrence to find a masterpiece on a floppy disc, but then it's a rare occurrence to find a masterpiece anywhere. In this digital age, it's going to become (IMHO) increasingly likely that valuable discoveries will be made on old digital media.