"It's not really helpful to speculate about the pebbles crushed to sand in the waves"

I'm not speculating, and didn't mention pebbles being 'crushed'.

The actual quote from Cecil Sharp is:

"Many, perhaps all of [the folk song's] most characteristic qualities, have subsequently been acquired during its journey down the ages, and represent the achievements of many generations of singers. Individual angles and irregularities have been gradually rubbed off and smoothed away by communal effort, just as the pebble on the sea shore is rounded and polished by the action
of the waves."

Sharp was describing improvement in the songs. Darwin would have said 'better adapted to their environment'. Pamela has already made it clear she was describing only 'change'.