What disappoints me, and presumably others, is not the sound(Steeleye was, and occasionally still is, marvelous), but the fact, which others have noticed, is that "folk tradition" as technically understood has vanished from the genre.

Probably inevitably.

"Trad folk-rock" is another minor concert genre in the musical spectrum. It is arduous to produce and perform, and it requires a team of professionals or semiprofessionals to do it. As is true of other music, the performers copyright their material and their imitators usually feel compelled to reproduce it (at least the words) as closely as possible.

It's hard to call something "folk" in any historically meaningful way if it can only be done right by well-equipped professionals. I suppose the accompaniment varies from performer to performer, but that's only one element of "tradition."