"throughout history a single line of melody is surely not what music is"

In traditional Irish music the single line of melody is the whole essence of the music, whatever that line suggests harmonically inside your head (which can be wonderful of course). And I don't think that too many people who know and love Bach's suites for unaccompanied cello would agree either. Yes there's a lot of double stopping but there is also a lot of not-double stopping too.