What you haven't mentioned in Rise Up Singing is the publishers, Quaker Song of Amherst MA. It's taken the US in the same direction the UK went in the 1920s, with the National Songbook, where a group of well-meaning but fundamentally ignorant middle-class Captain Mainwarings decided that folksong belonged to everyone, regardless of background, and so we had a generation of Gypsy Rovers. Fair enough, it gave something for the Revival to work off, but it's also put us 20 years back in the UK as we had to eliminate both that and the Garfunkels, with all respect to our American colleagues. It's the persistence of that second dynamic which gave rise to the "What is" thread, as it opened the door which had hitherto been firmly closed to original composition, and the Trad/Prog question, which thankfully isn't as virulent as it was in Jazz.
What is interesting is the crossover of showtunes into the populist repertoire. This is not new, of course, one of the earliest useful collections is the corpus of The Beggars Opera.
One interesting aspect of why people don't sing is because commercial pop is not singable. I've just trawled through the UK airwaves at the moment to listen to what is actually being played, and it has the following features: strong percussion, piano, nasal. Nothing is singable by the population. People don't go to church, so in most of the UK women don't sing, and so don't teach their children to.
All is not lost, however: I sing with London's Southbank VoiceLab and was roped in by Radio 3 earlier this year at the end of their Southbank Residency to lead half the bar in a rendition of Pharrell Williams' Happy - it's ballad-form, no instrumentals, and a relatively simple structure, and although the thing was impromptu (what happens when a choir over-hypes itself), virtually nobody copped out and some people really surprised themselves. Another event this last weekend made it possible for couples to get married almost en mass, saving the cost of huge receptions - VoiceLab was all over the shop, getting them involved. People want to sing, but have nothing to sing with. Perhaps it's time to get out of the back room of the pub and into the street.