Depends on the festival, imo. Some festivals could do a better job keeping trad acts on the main stages, but choose to keep them on the smaller stages. I can live with that, and it seems like a reasonable solution. On one hand, the festival needs to remain financially solvent if it is to continue, so having at least a few big name draws for the ticket sales is fine with me, even if the festival has historically been one where trad was common. Festival organizers do have some responsibility to keep trad prominent in the historically trad festivals. But I don't think they have any responsibility to keep them pure trad, especially if it means eventual bankruptcy.

As to singer-songwriter festivals, why not? I think they are fine too. There is room in the world for all sorts of music festivals. I'd need some proof that singer-songwriter festivals steal the audience for trad festivals--I wouldn't think the two would mix that much, considering the antipathy many trad purists feel towards the singer-songwriters.