I wonder whether we are in danger of applying modern assumptions and attitudes to earlier and very different times.

If a 50 verse ballad was unsingable, I am led to wonder why the writer bothered to make it so long. Surely a shorter, more singable version would have attracted more customers, and been less trouble to write. As Richard says, this was the fashion of the time, but that implies that these long and unwieldy compositions were in fact popular.

I think it is possible to exaggerate the "illiterate peasant" angle. Literacy rates steadily improved throughout the 19th century, and whilst many rural workers may have been functionally illiterate it seems quite possible that there was someone in their community who could read or sing a broadside to them. It is often reported that these singers had very retentive memories (a skill which illiteracy encourages) so it does not seem impossible to me that they could that way acquire at least a substantial part of even a lengthy ballad, from which they could then strip away the irrelevancies. And of course folk singers were not all agricultural labourers but included artisans and other skilled and semi-skilled occupations who might be expected to have a higher level of literacy.