"newly-written, introspective love songs"
These didn't really appear until the female writers began to work which was more in the 80's and subsequently, than in the 60's and 70's. Yes, the big audiences then were mostly into the songwriters and what the hell is wrong with anti-Vietnam war polemics? Those were the times and that's what they wanted to hear. If you go to the records of the writers of that time you'll find a lot of story songs,social comment songs, chorus songs and funny songs. Vibrant, relevant, full of energy many of them, and that applied to the performers who sang them too. When that began to disappear to be replaced with the introspective, respectful and frankly boring stuff, so the audiences waned.
Energy and passion is what young audiences like which is why some of the festival bands pack 'em in today, and why so many folk clubs have tiny audiences who massage each others egos and sing songs at each other. The same ones. Over and over again.