Old films have scratches, moldings, tears, color fade and many other forms of deterioration. They have happened, are happening and will happen. Nitrate films are combustible and prone to catch fire easily, while acetates are prone to hydrolysis, causing ‘vinegar syndrome’. Colors will fade. Sprockets may disintegrate. In many cases, no original negative has survived and only material available to us is a poorly handled dupes. Copying analogue data (images on films) always degrades the quality of the original, such as sharpness, brightness, grayscale/color balances and audio clarity. People often have said preserving the film prints and negatives is not a clever idea.
On the other hand, there are people who say photographic film is a proven medium for long-term film archival. You can watch Lumiere brothers cinema from 1890s with such clarity and beauty. No other media, DVD for example, is guaranteed to survive that long. You may have two or three reels of Super-8 home movies of your childhood, and you can project them nicely with profound amount of nostalgia. But your collection of 8mm video or VHS might have collected molding or something. Or it won’t start. Or images aren’t clear. Something wrong with that tape and you cannot figure it out. See? If handled properly, preserving cinema on films is the best way to go, they say.
Magnetic tapes do degrade. Information on magnetic tape is stored as magnetization of magnetic pigments coated on thin PET substrate. These pigments (magnetic particles) can lose their initial magnetization over a long time. Its coatings are made of ‘binder’, a glue, so to speak, and these can be degraded chemically. Also, magnetic tapes contain lubricants for better sliding against read/write heads and tape paths in a tape deck. These are oils derived from natural products, which attract moss and molding. It needs to be handled with care and stored in controlled environment.
How about DVD? It’s digital, so there is no degradation of quality upon duplication. It looks like a durable material, unless you make scratches on it.
When you say ‘DVD’, you have DVD-ROM (commercial DVDs) and DVD-R (ones you use for ripped copies). A DVD-ROM contains physical deformation (pits) created by a stamp, therefore it is quite robust. A DVD-R uses organic dye for creating pits and exposure to intense light (especially UV) should be avoided. It is probably not suited for long-term archives. There are DVD-RAM, DVD-RW and plus-minus and so on. Created for multiple read-write events, these are not meant to be long-term storage media. (Some people claim these RW disks have longer longevity compared to DVD-R.)
But these are only guidelines. There are always people who say ‘I have a ton of VHS tapes from 30 years ago and they look just fine’ or ‘I have many DVD-ROMs which won’t start!’ or ‘I transferred my Super-8 collections onto Blu-Ray disks and they are there forever without any degradation!’. From an engineering point of view, you have proven data of 100-year longevity for cinema films, 50-year for video tapes and 30-year for optical disks. Other data are derived from accerelated laboratory tests.