Don't forget... DVDs do and will degrade over time. Especially the writeable ones that everyone uses for backup.

External USB Hardrive storage is pretty reliable. The magnetic platters that they use are pretty robust. Even magnetic tape is pretty hardy.

Thumbdrives are prone to degrading. I think 10 years is where they max out.

Best solution, multiple backups. Backup to DVDs and USB drives. Keep copies stored someplace safe. Obviously "offsite" backups are the way to go.