1 - I am NOT a singer OR a songwriter.
2 - I know a few of the "folk" singers who also write songs. AND sing their own material.

That being said, these people usually ARE of the type who KNOW and sing the traditional songs. They are also people who have been brought up ON the Folk Music Tradition. Learning the songs from earlier performers (whether at their knees or on recording is immaterial).
For instance, would they complain about people like Ewan McColl, Stan Rogers, Archie Fisher, Tom Lewis or Dougie MacLean? These people are all among those who have their reperitoire of traditional songs, as well as new material they wrote themselves.

In a case of two of the singer/songwriters I encounter in the Halifax area, they write great songs, don't get the recognition they deserve, yet sing and write them any way!

The singer/songwriter of today will be, if they are good, the anonymous and forgotten writer of the folk song of a hundred years from now. I think we NEED to be able to hear these people.
I understand that some of these people write dreck, but EVERYONE knows Sturgeon's Law!