I've always thought that it takes 2 generations for rarity through age of a coin to kick in. For example, if my grandpa who as a young man in the 1910-20s had kept any coin in his pocket that was BU (straight from the bank) would be a valuable coin here in Australia (say at least 1000 AUD). Only a handful of modern rarities in my lifetime( or near enough) have escalated example 1955 Perth proof penny .. The rest boring