One thing that agents do is provide publicity. Whether this merely preaches to the converted or widens the audience is debatable. You can be sure that agents can only have a positive influence- to what extent they encouraged a revival is hard to say.
The one difference between today and the sixties is the lack of folk exposure in the media. The Corries, Robin Hall and Jimmy Macgregor, Julie Felix, The Spinners to name but a few, they all had exposure regularly on prime time TV back in the sixties and groups such as the Strawbs, Pentangle, Steeleye Span all got into the hit parade in the 70's
Such things no longer happen today, so perhaps we owe a lot to agents for keeping the genre in the public eye. Again no real statistics to support the view, just a gut feeling.