I'm not sure about this Mike. There was definitely low points after (let's just say 1980 for now). I would date a gradual decline from the days of folk rock when amplification became necessary and places without it (and sometimes fairly sophisticated gear) started to close. I am obviously thinking of clubs here.

What kept it going were those who still offered clubs and those who still went. Martin Carthy, John Kirkpatrick, but I reckon there were not many folk singers or instrumentalists (however defined) who "started" as professionals in this period.

I would argue that the "development" is a qualitative change, from a "vocal" to an instrumental revival. And that's one we haven't had before.