However what we have found is R2R and cassette tapes are good for 50+ years.

hmmm...... VHS tapes that were used extensively by the record industry to archive many famous and successful digital audio recordings was found to suffer when they revisited them. The medium that bound the ferrous particles had oused from the edges and coagulated. As the tapes unwound it ripped off the protective layer and some of the ferrous layer. They tried baking the tape with moderate success. Decca used a propriety system so didn't follow the rest of the industry and their digital R2R system didn't suffer in the same way.

Cassettes inhabit the same area. And "print-through" is a known degradation with analogue & more prevalent in cassettes. Because the tape is wound & thin. But as I inevitably say when the HiFi is questioned - "how good a quality is a blank recording?"

in my application CDs & DVDs are a transfer medium. A review vehicle. A discussion point. Who knows what legacy equipment they posses. The historian receiving the DVDs is nearly 80, his wife does the computery stuff and she merely knows what she knows! My project is to get his stuff and voice to our local museum and Gloucestershire Archives where they have experts in digital archivery wotsit.

And at the end of the day his collection of slides, postcards (& slides thereof) and published books, will last longer still, but not his voice and extensive knowledge.