An argument that there are fewer folk clubs because clubs started to include music not counting as 'folk' in some definitions, and so people stopped turning up would seem to be specious, as if there were a demand for that sort of music the clubs, logically, would have continued to provide it. Why would they stop providing stuff that people wanted to hear? Even a weaker form of the argument, to the effect that such clubs did not provide exclusively 'folk-in-a-particular-definition, but drove people away by also providing not-folk so horrid that people were deterred from coming seems difficult to sustain.