It depends on the drive and the type of drive, but generally they actually start to suffer bit rot after 6-12 months if they’re left without power at all. DVDs also suffer from bit-rot on short time scales, especially if stored improperly. Also migrating data or having a copy of it on a hard drive is easy, copying DVDs is legally gray, difficult, and they really only make sense as a delivery vehicle for the file to a drive or tape (if you prefer something genuinely long-term storage ready).