You've now tied yourself in the curious knot of claiming that there are some paintings created after 1500 that are
exactly as good as the best works of the earlier era, but no better, and that that's what you meant to say all along.
What's particularly weird about this argument is that is would not affect your essay at all to concede that, say, the period called the 'Golden Age of Dutch Painting', or the Golden Age of Spanish Painting, or one of the several other periods that art historians (inconveniently for you) commonly refer to as Golden ages of painting produced better work that the somewhat fussy and stylized paintings of the Renaissance, and that you just got excited because you really like Leonardo.
But you seem unwilling to back down from any claim once you've made it in print, no matter how silly ("Cezanne couldn't draw!"), and so we get this peculiar kind of weaseling.
Western art, like Western music or mathematics, is a cumulative enterprise, but you've done the equivalent of claiming Pythagoras was the best mathematician ever, and as evidence pointing to the fact that no serious historian of mathematics has ever said in print that Pythagoras was a worse mathematician than Euler or Gauss, when it would not occur to anyone else to think in those terms.
Apart from the fact that this is just blatant argument from authority ("no art historian would disagree with me! Not even this one, who just disagreed!"), it also a disservice to your readers, who are taking it on faith that you know what you are talking about. As Aaron's correspondence demonstrates, when someone finally does corner an art historian, and bends the poor guy's mind into thinking of the history of Western painting in the same weird categories you do, he'll politely say that you're completely full of it.
Not being an expert in philosophy, or economic history, or the trade union movement, but being a longtime student of your writing style, I suspect the same thing would happen in those fields. I just happen to know art history, so when you write about that I can poke the holes myself.
You've been coasting on the fact that most of your readers are technical people who will take the analogies you strip-mine from the humanities at face value because of their respect for you as a clever guy. You can probably continue coasting on it for the remainder of your career.
But I think you'll find that a little more humility, intellectual honesty and the confidence to abandon the occasional overeager metaphor, rather than trying at all costs to defend it, would make you an even better writer than you think you are now. Maybe not a better writer than EB White, but possibly exactly as good.