CD longevity?

IT guys like me are always harping about how you should back up more frequently.  I've probably said 100 times, "CDs are cheap, and backing up your files to a permanent CD is so easy it's foolish not to!"

I still advise that, but one side of this issue that doesn't get talked about a lot is: How long will those CDs last? 

I can picture some of you thinking, "Wait, don't they last forever? Isn't that the point?" 

They don't last forever. A few days ago I needed a really old file, from a CD I burned in 1999 - when I first started archiving materials to CD. No dice. The disc fails on every computer I try. That stuff is gone-daddy-gone.

Now, part of it could be that mass-market CDRs were just evolving, and CD burners were slow, cumbersome, and error-prone. This is the era when the phrase "burn a coaster" was coined. But I'm just not sure. How long will CDs I burned a year ago last? What about DVDS? I'll have to look into this a bit further. Maybe 20 years seems like a long time, but my photo library is digital - not print. I'd like my kids to have photos after I'm gone, which means (hopefully) that my photo CDs need to last at least another 40 years. 

I do know that CDs and DVDs need to be stored in reasonable environments - away from sunlight and moisture. And I have audio CDs from 1990 that still work (I suspect home-burned discs are more susceptible to problems).  And there is definitely a difference in quality among brands. One time I got a spindle of some no-label blank DVDs with a DVD burner I purchased, and they were horrible. It has to do with their layer and substrate construction and materials. 

It's also true that some brands are really the same disc just packaged by difference companies (Apple 4x DVD-Rs are very reliable,  but about 1% more expensive than Maxell 4x DVD-Rs - and they're the exact same disc.) 

Anyway, what to do? I guess if you have data you want to hold onto forever, there's no avoiding a mass migrating of all of it from one (older) medium to another (maybe newer) medium. Some big corporate data centers have policies where they routinely copy all of their archives to new tapes, for example, to keep the shelf-life age of their data as low as possible.  Maybe by the time it really becomes an issue, there'll be some sort of fool-proof holographic storage that holds multiple terabytes and never goes bad. 

Disk-based storage might be an answer, but hard drives fail too. I guess the only foolproof way would be to keep a RAID system running with swappable drives, so that as drives die out you can replace them and always have a reliable destination for your backups and archive files.

Don't think about this too hard, though. You can really make yourself nuts over it. "OK, I've copied all my old archive CDs to fresh CDs. But NOW I should make a third copy to store in my safe deposit box..." and on and on and on....