Just had this thought, and realized all my backups had been through a flood. I wound up with several damaged disks, but was able to get all the content off of them, damage was to the outside edges, and most of them didn’t have much data written.

I have a Blu-ray burner, but I’ve never burned one (only a couple DVD’s). Probably never will.

Yes, the older ones of the rotating backups are still readable.

But that’s not even the actual problem nowadays: CDs and DVDs were nice when their size was still relevant in comparison to usual amounts of data. The real problem behind their decay is that we are lacking a widely available, properly scaled backup solution for more than a decade. So the mean reason people have now unreadable optical data is that they stopped thinking about it a long time ago for an utter lack of options.