About the Middle Ages, does any of the folks who pretend that it was as culturally prosperous as what came before, Rome, or afterwards, Renaissance and modernity, know any writer or philosopher from this time? Literature and philosophy might be an arbitrary choice and perhaps I am just too ignorant but I don't know any writer between Cicero and Shakespeare and Thomas of Aquin plus Meister Eckhardt are the only Middle Age philosophers (or rather theologians which hints at one cultural problem) which come to mind.
Of course the pop cultural notion that the "Dark Ages" were all about burning witches and people believing that the world is flat is nonsensical ... but so is the notion that it was a time like any other.
About why this might not be culturally great times, the point which has already been made, 'increasing commercialization of culture as a result of increasing corporate power', seems fairly objective to me.
I wouldn't go so far and use this as an argument for cultural decline though as there is still plenty of good stuff. It is just harder to find amidst the increasing amount of commercial crap.