Most of the people mentioned here as featuring in the book are still performing after a career lasting decades. As well as inbalances of geography and professional/amateur status in the selection, is it also unbalanced in that it selected people who stuck with it?

If you're going to talk about decline, it would make sense to interview people who contributed to the decline by moving into a different career or resorting to a different kind of entertainment.

Just spotted this upthread:

Ewan MacColl [...] influenced people to look to their uk roots, in some ways that was good,but it was never going to be of interest commercially.

Sure about that? The Beatles developed a repertoire of pop music built on the British music-hall idiom (and perhaps to a lesser extent the songs of Noel Coward) - they were every bit as much an indigenous product as MacColl with the Radio Ballads. And it wasn't a liability for them.