As the actual question was "What If", my thoughts would center more on how much the rock scene has been influenced by the previous generation of musicians as well as the current sociological and political environment of the time it lived in. So let's say electric guitars, amps and other gear were available in the 1930's, and there were already bands playing what one would call 'rock music'. I guess Woody Guthry and the likes would have discovered the music and 'this land is your land' would now be more like 'If I had a hammer' or even a power ballad today mostly known for its BonJovi cover version. But without the rise of teenage culture and record technology, could someone like Elvis' elder cousin stake his claim against Frank Sinatra or even Glenn Miller?
My own guess is that there are too many unknowns. Even if you would magically both transport punkrock and heavy metal 15 years back to the time of the Vietnam war, there is no guarantee that you would have grunge in 1977 instead of 1992. Yes , there would be some new kind of music in '77 but without the feeling of a generation X, it would be just as far from OTL's grungerock as it would be from OTL's 1977 Queen.
My own guess is that there are too many unknowns. Even if you would magically both transport punkrock and heavy metal 15 years back to the time of the Vietnam war, there is no guarantee that you would have grunge in 1977 instead of 1992. Yes , there would be some new kind of music in '77 but without the feeling of a generation X, it would be just as far from OTL's grungerock as it would be from OTL's 1977 Queen.