'common-sense contradicts this'
Really? Who's common sense? Please explain.

'The bulk of our knowledge of the repertoire does not pre-date the beginning of the 20th century'
I think I understand what you're trying to say but I disagree anyway.
As for the ballads, what Child has to say on the ballads (not always right) has still not been surpassed even in the 21st century.

As for the English repertoire 89% of it existed on commercial sheets of some sort before the likes of Sharp came along and indeed that 89% is the earliest extant form of each item in the canon, whether that be 16thc, 17thc, 18thc or 19thc.