"The tunebooks of Thompson, Playford, Walsh, Wright etc. probably did more to transmit the tradition than aural sources ever did."

I agree with that. Being heavily involved in the Village Music Project I know that the vast majority of the tunes in the MSS had previously appeared in print.
However, that doesn't necessarily mean that the tunebooks were read from in the wild. Judging by the condition of most of the surviving MSS, and the illegibility, and the fact that they were mostly written by young people, they seem to have been used as an ititial source for learning at home.
The other possibilty is that the non-survivors were very legible and consequently read to bits. :-)