We know with some accuracy the survival rates of very rare stamps for instance. The 12 Penny Canada (Scott #3) a perfect copy of which was just sold for $488,000 has been counted carefully since the early days of collecting. There are 120 surviving copies. The number of copies sold by Post Offices was only 1450 so the survival rate is nearly 10%. If such a survival rate existed for US #1s there would be 360,000 stamps in collectors’ hands – ten for every member of the American Philatelic Society – a number I believe to be high by a factor of three or four.
Perhaps the answer is in a story my Grandfather told me in the 1960s. He remembered being told of great mail house finds in the late 1880s and 1890s. Old time dealers told him, (when he was a young man in 1920) that they went through these old letters and unless a stamp was a rarity (like the 12p Canada) it had to be in very nice condition or they would just tossed it out. US #1s were so common and inexpensive that unless they looked quite nice they could easily be discarded. This would account for the relative high quality of most specimens (a very high percentage are four margins) though it doesn’t help us figure out how many still exist in collector’s hands.