I don't think the article would consider any of the people you listed polymaths. One of the article's criterion for being a polymath is that you have interests in diverse fields. A musician who can play several different instruments isn't showing much in the way of diversity. Neither is James Cameron, nor Louis CK, because the examples you cited all are tied together very naturally, and I would argue that being a great director probably means you _have_ to be a good set/costume designer too. And you can't just be really good to be considered a polymath--you have to be an expert. Thomas Young, from the article, showed that light was a wave in addition to a particle, _and_ contributed immensely to linguistics through helping to crack the Rosetta Stone. Making significant contributions or developments like Young's these days isn't possible. With the article's stringent criteria, I'd agree--what polymaths we have today aren't great compares to the ones we used to have, and they're just going to keep getting worse because it's hard to be an expert in anything intellectual nowadays, and it'll only get worse.
I might agree re: Louis CK. I wouldn't agree re: James Cameron. Set/costume design is an entirely different art form from directing; I was actually very impressed when I read about his early work designing for films before he became a director. Saying the two are similar arts would be like saying that, I don't know, working with Photoshop and Illustrator is close to being a marble sculptor.
I'd also cite, say, Ricky Gervais as an instance of a person who is pushing the boundaries of multiple fields at once. His directorial style inspired a dozen other single-camera television comedies (including Arrested Development, which I think is the king sitcom); his acting style and his "theater of embarrassment" similarly begat actors like Michael Cera, and a wide stripe of other comic actors who play off deadpan silence. His writing style, meanwhile, especially for the original Office and for Extras which followed it, was incredibly bleak and devastating, and simultaneously very heartwarming. He had a sympathy for his characters which very few TV shows had. I think his particular blend really set a trend for a decade of television that followed.
I think the problem is how we define "expert". If you mean somebody who is definitively at the top of a field, then, yes, you're right, we don't have many people at the tops of multiple fields at once. That's because it's hard: We have more competitive fields now than we've ever had before. More experts in every single thing. But I'd argue that we probably have just as many people operating on that high a plain of expertise in multiple areas. It's just that when you've got so many, it becomes very, very hard to notice every single one, and so the bar for "exemplary" gets incredibly high. It's a bias in the data that returns more dismal-looking results.
Addendum 1: Unfortunately I can't name many people in the sciences, because I'm as far from a science person as you get. I'd be convinced they're there, though.
Addendum 2: I don't just mean a musician who plays multiple instruments. I mean a musician who plays multiple aesthetic styles with radically different philosophies. There are guitarists who can play funk music and flamenco and free jazz and modern classical. If you know anything about the cultures and styles of any of those four, you know that the difference between playing each isn't just technical ability. It literally requires playing the instrument in ways that are anathema to any of the others.
You might argue that just because all four styles are classified as "music", they are too tied together to count as diverse; I'd argue that you're wrong. They're about as similar to each other as physics are to linguistics, which is to say that while there are similarities, they're simply too far apart to be umbrella'd together without doing each a great disservice.
Sure, set/costume design is different from directing, but they fall under the same umbrella of having an interest in making movies. James Cameron is not a polymath for the same reason someone who gets a Nobel in physics and chemistry is not a polymath. If someone gets a Nobel in literature and physics, that's being a polymath. Your music example also isn't great. I, and several other people, know how to do a wide variety of dances--square, Irish folk dancing, salsa, waltz, hip hop, lyrical, and so on. It does not make us polymaths. The differences within a discipline are pretty big, true, but the difference between disciplines supersedes those. Someone who programs and dances is jumping a much greater distance than someone who goes from ballet to hip hop.
You seem like a talented and energetic person, so kudos to you, and this discussion is ultimately about semantics, which is uninteresting. I still think you misunderstand what breed of intellectual the article is talking about, though.
