Necessary, because if it were not true — if digital music IS distributed with innovative visual art — then we cannot conclude that rock music has “nothing” going for it. Rock music would still have innovative visual art, as it did back when it was released on LPs.

Not necessary, because we know that digital music “has almost ended the production of rock LPs.” That tells us that there are almost no rock LPs being produced. Whether rock LPs are “most” of the small number of LPs that are produced doesn’t matter, because there are still almost no rock LP’s produced.

Not necessary, because the author’s reasoning doesn’t rely on any belief about LPs from other genres.

Not necessary, because the argument’s reasoning is based on the fact that rock music is almost never released on LPs today. So whatever is true about the LPs that are produced doesn’t matter.

Not necessary, because we already know from a premise that rock music is musically bankrupt and socially destructive. Even if rock music is not any less musically sophisticated or destructive today doesn’t change the fact that it’s still not musically sophisticated and still socially destructive.