>> ...how many people ....... could sing a single verse of any of those - particularly 'American Pie'...

You could say the same thing about Mattie Groves, The Nutting Girl and even Lincolnshire Poacher. <<

You could indeed, if we were talking about 2008. However, to take one of your examples, "Mattie Groves" was collected at least eighty times by song collectors in the twentieth century, with informants (many of them North American) regularly recalling twenty or more verses. Bearing in mind the extremely patchy nature of song collecting, and the fact that by this point traditional singing was already in decline in industrial Britain and North America, it seems reasonable to assume that at some earlier point, rather a lot of people knew rather more than one verse of "Mattie Groves". Which I think is what I was trying to say before.