Silver San Juan was printed by the offset method, and plates for this type of printing are routinely disposed of, but usually the stripped up page negatives are retained. More important than the plates, it is likely that the printer destroyed the negatives at some point when no second edition came forth. Trade customs in the printing industry, still in effect today, call for the retention of plates, negatives, etc., for 30 days after acceptance of the job by the customer, and after that, material is subject to a holding and storage charge unless otherwise negotiated. This book could be digitally scanned and reproduced today with no noticeable loss of quality.
The plates in that case as such are no big deal. Books that were printed letterpress, like the original printing of Denver, South Park and Pacific, used metal type and individual photoengravings. That book was printed by World Press in Denver and as the printing progressed, the Linotype slugs used for text and captions were melted down for reuse as World Press had a limited supply of metal. That poses a problem if a second printing were to take place, because in a sense, the type was "destroyed" but could have been reset. The plates survived, however, and I have the photoengraving of the Jefferson depot used in the original printing.
The latest shift in printing technology towards digital storage of type and illustration means that one CD could hold the entire book and that eliminates the need for plates and negatives. Letterpress is akin to steam engines, offset is the diesel of printing, and I'm not sure there is a railroad analogy yet to the digital age.
