Re the Grateful Dead I'm sure they had hand in introducing (and reintroducing) their audience, myself included, to types of music they might not otherwise be aware or had forgotten about like Country,Acoustic and Electric Blues, Bluegrass, Old Time, Jugband etc. that might otherwise have been "doomed to oblivion. I thought this was especially true in their post- psychedelic pre-hiatus period which roughly spanned '69 to '74.