I think a lot of copper drek that was once plentiful will begin to dry up in high grade at some point. How long have these books been lingering in discount bins, handled/stored poorly due to their red-headed stepchild status?? Not to mention the fact that a lot of people realized many years ago that they couldn't get squat for them from an LCS or ebay, and just tossed them.
Some books will take longer than others obviously, but I already have a tough time locating HG, NM or better copies of lots of 90's books, especially 2nd and later printings, and issues in the higher numbers of a title's life.