The near ubiquity (in female singers of folk and the music recently called folk) of the wifty-wafty voice can't wholly be blamed on amplification. Obviously it is enabled by amplification, but the total ubiquity of amplification in rock music has still left quite a lot of female rock "belters".

Indeed I would think it was quite a lot later than the revival. Yes, the good dame blamed above was a bit restrained, and likewise the puzzlingly revered Anne Briggs but right through to the end of the folk-rock scene there were numbers of strong female singers, whereas now the nuber must be down to the Waterson-Carthies, Maddy Prior and June Tabor: that's about it AFAIK.

As for "blame" - yes, I think that's a fair term to use. To get back onto my hobby horse I see the baby-voice as consistent with the societal image of the woman as child and potential victim, consistently with depilation and the modern equivalents of foot-binding and hobbling.