If I'm reading the above correctly, the responses are that performers do not generally make/register their own arrangements with the intention of claiming royalties, because the return from the royalties does not make it worthwhile. That's certainly relevant, but slightly different from what I meant to ask. Rather than saying "maximising income" I should perhaps have said "minimising expenditure" - thinking that the costs of producing a CD would be less if the performer did not have to pay royalties to a predecessor.

A club singer will happily sing song X, based on the arrangement or version as it came from whomever he/she first heard it. This might well be different from the version originally collected at the turn of the century, or that as encountered in the singing of singer Y in the 1930s, different again to group X in the 1970s. Yes, he/she will introduce changes to match personal style or ability, but changes based on this kind of approach will have spread slowly in the past - and would nowadays were this the only route for change. Perhaps wrongly, I would expect a paid performer to feel a greater need to produce something distinctive rather than a repeat, and wondered if the influence of copyright was driving this to a greater extent than otherwise would have occurred.

I think George is right that other influences affect this: particularly the modern ease of obtaining large quantities of music in such wide varieties. But I don't want to widen the question.