Firefighter- You're right, the life of a DVD or VHS tape can be shortened by poor care.
But I think what crisi meant was that IF a DVD is taken care of, it will not degrade in quality. Only a scratch or an actual break will affect it, and since a DVD is read with a laser, it shouldn't get scratched with proper care.
With a VHS tape, or an audio tape for that matter, the quality degrades slightly with each use because the reading mechanism relies on physical contact, and the tape experiences wear.
So not only does a DVD look and sound better than a tape from the start, it will retain its quality while a tape will not. So a DVD can be "rentable" for much longer than a tape.