I think we're talking at cross purposes.

Irish folk music certainly existed before the Clancy Brothers became major recording stars. However when an act becomes so popular that it is seemingly ubiquitous - it becomes a force that ordinary gigging musicians have to take note of in the perennial quest of putting food on the table.

Rhythm and Blues music existed in America before the Beatles, the Stones, the Yardbirds and the Animals - but the careers of muddy Waters, Nina Simone, John Lee Hooker, Chuck Berry etc started doing bigger business after the British invasion.

Such trends are a simple observable fact.As Doc Watson said, when something like the Beatles comes along - you run with the idea.

Similarly I don't think Wald has any need to defend.. the truth of his contention is the sudden proliferation of the terms like ragtime and blues as titles for songs makes his point. as does the appearance of cultural like Shakespepherian rag in TS Eliots The Wasteland.

blues /ragtime - it was in the air that people breathed all over the western world at that time. musicians were hardly likely to miss the trend.