At the risk of talking into thin air, I think there are a variety of factors which determine "greatness". Taking into account external factors and that music doesn't exist in isolation -it sits within a specific time it was made and is both influenced by society at the time, and influences society. Does greatness mean it has to be recognised by the many? Where does originality come into it and the impact on wider society? Is the impact felt less now by contemporary music because accessibility and originality is more diluted? What about inspiration? Some of the bands mentioned already existed within a boom in culture arts etc. The fact humans "express" themselves online these days - does this mean, such things (as political discontent for example) don't get expressed? What's the inspiration for musicians today?
I think one measure of greatness is whether or not people continue to listen to their music years and decades after their prime time
Yes I agree and there are some artists who were largely unknown in their time but have gone on to have a huge influence in the years/decades afterwards.
