I have one final footnote concerning the value of works on paper vs more archival mediums.
What Ivan Karp told me back in the 70's may have been true at the time, but it no longer applies to today's art market. The work we now see in cutting-edge galleries is often ephemeral and fragile.
For example: Raymond Pettibone's displays of slight drawings on cheap paper pinned unframed to the gallery wall, or other artists who use substrates that include cardboard or butcher paper. Straw deteriorates and falls from the surface of Anselm Kiefer's paintings, and pieces of broken plates routinely fall from Julian Schnabel's early work.
However, a work done on acid-free rag paper can have a long life if treated with care.
After all, Rembrandt’s drawings, DaVinci’s coda, Durer’s etchings all survive after hundreds of years. Rule of thumb: if you own a work on paper make sure its framed with acid-free backing and never let it be exposed to direct sunlight. In fact, nowadays, huge sums of money are often paid for works on paper.
Since today’s contemporary artists don’t seem concerned with the archival qualities of their work, I forsee unlimited future employment for an army of art restorers. Only time will tell.