^^ Right, exactly!
^ Yeah, maybe it's just me, but with literature especially I find it shocking to say "oh there was nothing anybody remembers between Cicero and Shakespeare." That's 1500 years that we have a large number of records from!
Of course there was literature produced in this period. But do you still read any of this stuff (language is no counterargument, Tolkien did e.g. translate Beowulf into modern English)?. Take theatre, as far as I know before Shakespeare there were only mystery and morality plays who were probably of far lower quality than the few remaining plays of classical antiquity.
Same with philosophy. You don't miss anything when you jump from Greek philosophers to Spinoza unless you really care about all that theological-philosophical Middle Age stuff.
You have admitted that the quality of art and music is subjective
To some degree, yes. For example claiming that one Trek story is objectively better than another would be nonsensical.
But it is safe to say that Hitchcock movie is objectively better than a Michel Bay movie, that a Mozart symphony is objectively better than a KRS-One LP or that Moyers & Company is objectively better than Big Brother.