I was looking for a quote from Jim Carroll before, but couldn't find it, now I have. So my apologies for being late in response.

"Cap'n,
The groups you mentioned - and others - came and went without leaving anything of lasting value to traditional song."

The first point is that it is a bit difficult (read impossible) for any recent group or singer(s) to affect traditional song, which by definition predates them.

If you consider that rather a restricted definition, then my second point is, I think, better. What these people left was an audience. New singers, musicians and listeners have come to the tradition via the songs of Steelye Span, Fairport Convention, etc. You may consider the output from these groups to be "tradition-lite" at best, but without them the current folk scene would be even smaller than it is, and I don't see that as being beneficial to passing on the tradition, however defined.