Without the resurgence in interest that Child (and others, of course) partly instigated and had such a seminal influence upon, many subsequent collectors and scholars would never have embarked on their work, which eventually gave rise to the Revivals of the 20th century, and in turn led to a great many traditional singers being "discovered" and acquiring audiences outside their small, immediate circle of acquaintances.  Doubtless, many would still be singing (though many would not, having only begun to sing again when they found that there was actually somebody who was interested), but we wouldn't know about it.  Traditional singers do not live in a vacuum, and are no longer, as early collectors saw them, unlettered peasants living in remote areas with little contact with the outside world; many of them have indeed learned a proportion of their material from books and records, many of which would not exist but for Child's influence.  I see nothing incredible or even mildly contentious in my comment.

Although the texts Child published were taken from other collections, many of these were obscure and inaccessible, or available in "improved" (that is, chopped about and re-written) form only.

So far as defining ballad goes, it should be remembered that such definitions vary with time and context, and that "ballad" and "folksong" are not necessarily the same thing.  Child stuck, for his purposes, to the narrative form, which is and was perfectly proper.  It is fairly pointless criticising him, with the benefit of a century's hindsight, for not being more inclusive, and I'd think that to try to examine his achievement using different terms of reference from those he actually worked with is unlikely to achieve much.