I'd already established my riding/life style, so I had a good commuting bike by 1970. Didn't stop me from buying a nice Italian factory bike in 1973, which I still have. My take on the falloff is that simply, most of those who were aiming to make a bike purchase - those who had incorporated alternative transportation into their life - purchased in these boom years, and those to whom such a purchase simply flowed from their disposable income frequently found that moving about with these machines was too much work. This latter group simply hung them up in their shed or garage.