Longevity? As long as I keep my iPhoto Library backed up, that's all the longevity I need. With regular backups, longevity for digital pictures is pretty much forever.
It's been suggested that in centuries' time this time will be looked at as a "dark ages" because there'll be little remaining evidence depicting events of this time.
The photos you've taken are recorded and kept in digital form. But that file format will not always be supported. Some day, way down the road in the furture, every computer will look at a file with the extension "jpg" and not know what to do with it.
Someday, way down the future, you'll be long dead meaning every photo you've taken in digital form will be lost. You've kept it stored on hard drives which degrade and crash and don't last forever, on CDs -a format that won't always be supported, on a flash-drive another format that won't be supported, you have your photos on an iWhatever -a program that won't always be supported.
This is a pretty big problem. And even if you printed out those pictures that ink-on-paper won't last forever. Infact, it's pretty unlikely it'll even last your lifetime.
Taking a real picture on film and getting it developed produces a picture that is permanent. A picture that is formed on the paper using chemicals that etch the photo
into the photographic paper. Permanently. Now, sure, even that can degrade or become ruined if not treated or cared for properly, but ink on paper is much more vulnerable and even fades.
It's not a permanent form of photography and that disturbs me. Future generations are going to wonder what life was like in this time and they're not going to know because all of our photography either degraded over time because of its poor medium or pictures are locked up file formats/mediums they can't access.
Nothing beats a good, old fashioned, photograph. Which Polaroids have over digital photography. It's more permanent.