Cost is one thing, but does the capacity not bother you too? DVDs are so small in comparison to BDs. Not to mention the time you have to waste switching discs (or reading/writing them, if relevant).
It also means that the failure of a whole disk has a lesser impact on the whole.
Edit: that said, I do have a spindle of 100 CD-R that were given to me. I haven't had the motivation to use them up yet.
It was mostly cheap CD-R from that period, and frankly you needn't wait 20 years for them to fall apart, they'd start failing under 5. Quality CD-R from good brands survived just fine.
But I'm wondering when it'll actually start deteriorating and ultimately fail. So far it seems to still be OK.
Your Ubuntu 6.04 installer is probably burnt onto a CDR, unless you bought a pressed copy.
The AOL dial-up CDs are most certainly pressed CDs, and would have a far longer life.
I've seen bit rot in as little as 5 years on cheap CD-Rs.
I've also been backing up personal photos to DVD annually for quite a few years. Each disc contains encrypted Duplicity files, and a PAR2 archive to assist with any deep failures of the on-medium parity. I also usually include a disk or two of pure parity archives, to handle a total failure of any disk or two in the middle.
Each year I check the price of DVD vs. Blu-ray and when factoring in the cost of the burner DVD seems to stay just on top for $/GB. It doesn't help that I've still got spare DVDs lying around, and people basically give away the blanks these days.
This is not my only backup strategy, but it is the only one that is cold, read-only, and offline.