It assumes one moment in time, the mid-late C19th, was an uncompromised window on common popular music and the recorders of it were impartial.

How does it assume this?

It offers a closed world to all intents and purposes

Two questions. Firstly, how does it do this? Secondly, I am personally of the opinion - although this is nowhere in the 1954 definition - that there probably won't be any more folk music in Britain or Ireland; the conditions under which the process described by Karpeles could work don't obtain any longer. But if I'm right, so what? What do you object to in the idea that the folk process as described by Karpeles is a thing of the past? I'm genuinely curious - this is a real stumbling-block for me in understanding some of the arguments made here.

It contains interesting ideas, a challenge to authorship in a period marked by personal rather than community expression but takes the point to absurdity insisting anonimity is of the essense, not a consequence.

It doesn't do this, as has already been pointed out on this thread. Anonymity is precisely a consequence, not of the essence.