A few things, since Joel's original question was generational. So I guess we are talking 80 years plus.

I have read about Data Degradation, if you are saving in multiple locations how much of a problem would it really be over the long haul?

How about archiving photos? Of course we all make JPG's for viewing, but how about our digital negatives and raw files?
Who is to say that Windows 20 will even be compatible with a program that would handle CR2 files. Is there a base negative file we could convert RAW in to that would for sure be available in the future?

Unlike film negatives of the past which was put on a medium and remains on a medium, our medium is forever evolving and changing. Negatives of the past still exist as tangible objects. Where are photos exist as data on a computer that is continually upgraded, and to save our files we will continually be saving them on new computers and storage devices. The oldest digital pictures I have saved are from 1999 and they have been on at least 5 or 6 different machines, probably more. I have video from the same time frame shot with a webcam, I am unable to view it now. How will our digital pictures fair in 50 years after going through dozens if not hundreds of formats and memory transfers?