Say you write a novel. It's about a man who catches this big fish, but that's not important right now. What's important is from your perspective you're just telling a basic narrative: man, nature, fish, struggle, fish in boat, end. Your publisher loves the story and out into the public domain it goes.
What happens next is both magical and frustrating depending on who you ask. First time students seem to have the same perspective as the author: it's just a narrative. There's a sky, I've seen a sky before, oh it's a cloudy sky, okay let me reassess, ok got it. Move on.
Now comes the pesky English instructor who insists there is symbolism and subtext in the narrative. The clouds are indicative of rain, and rain usually suggests something's going to happen to the characters. They get wet? No Johnny, you're obviously not getting it. Rain is foreshadowing to the story. Oh and that catching the fish business? Well if you thought that's what the story was about, you're clearly wrong about that as well. The catching of the fish is a metaphor. To find out specifically what metaphor, let's take a look at the author's life and look for clues. Maybe we see something that was relevant during his time like Orwell did with Animal Farm. Maybe it doesn't matter —- maybe the story is so universal that it can apply to any time period, or toward any reader —- maybe, the story is whatever you want it to be.
Apparently, so long as you can make a compelling argument, any story can be about anything. Which is magical when you think about it — that if you're clever and you look for patterns, you might unlock something, discover some universal truth that is important to the personal or greater good.
That's great except those authors never intended for you to see anything beyond the words they put on the page. In fact, there's an upsettingly large number of authors who have flat out said, no that's not what I meant. Don't believe me? Check out this Metal Floss article:
So where does that leave us? Well fortunately modern times has procured the perfect answer and it's called 'Screw that author'. See, authors, believe it or not, are people. People are governed by mental processes, and as memory goes, it's one of the most flawed and biased systems we have available. As Einstein said, "Memory is deceptive because it is colored by today's events." In short, we can't trust ourselves because so much of our lives that we would like to think are factual are instead closer to opinion. With that in mind, when an author writes a story, it is already colored with his or her subconscious thoughts, bias, and metaphors. Adding conscious narrative structure: the need for this to bookend with that, conscious foreshadowing, etc, the author creates a torrent of personality in even the most basic of stories.
As readers of a story written last week, last year, last decade, we are neither unable to crawl inside the mind of the author nor transport ourselves to that time frame. We have no idea what guided that author to write what was written, or draw what conclusions were made. As a result, it's up to the reader to figure it out for themselves. A clever or smarter reader may draw amazing parallels or uncover an amazing conclusion no one else saw.
When a reader finds out the author never intended that, or a director said, no that's not what I was trying to show, does that mean it's over? Should I ball up my fan theory simply because it contradicts canon? No. Screw that author. Your opinion is just as valid as his. Just because he wrote it, that doesn't mean he knows what's best. (Yes, I'm looking at you George Lucas). People go to therapists all the time because they don't know themselves and they are looking for answers; all of us look for validation or make changes to our convictions because of this reason or that. As far as movies go, it's the same thing but in spades. Yes there was a screenplay written by someone, but there's also an actor who inhabited the role. Who's right? There have been many times an actor, just got it wrong and it doesn't have to be limited to the whole movie, it could have been one word of dialogue that with the wrong lilt, changes the whole context of the whole. Directors certainly don't have the final word because although they do have an amazing amount of control, there's also a great deal they don't: they rely on a screenplay, the way a film is edited affects the final work, and so on. So although it's fun to listen to actors and directors talk about this or that, don't let them override what you got from the movie because that's what should be the final say.
At the end of the day, a reader (or viewer) must draw their own conclusions to the final work that is presented to them. The fair thing should be to judge something based only on what was given —- in other words, it's unfair to compare Man of Steel with comic book Superman because the two are independent creatures. If you want to talk about the history of Superman, that's different, but if you're going to judge Man of Steel, judge it within the confines of those 2 1/2 hours.
You might go back to my original premise and go "screw that guy", I can judge anything I want on whatever I want. Although that's true, I think that's a little out of bounds. You're bringing in motivations because you feel all heroes operate according to a certain trope; however, this particular body of work doesn't demonstrate that motivation at all, so doesn't that mean the argument is a bit manufactured?
Anyway. Back to the original premise: is symbolism a modern construct? Yes and no. Authors once upon a time probably intended to write stories with specific purposes and nothing between the lines. However, as said before, authors being human were always guided by their times, their memories, and their prose. Us consumers of those bodies of work will always see them through the prism of our own times and although we can try to put this or that aside and see the work for what was intended, is it important enough to actually do that? Probably not. Modern authors know symbolism and context exist; there are more red herrings and set-ups in an episode of Agents of Shield than a typical 20th century novel. As a result, modern works, movies, books, video games, are full of complicated story arcs and characters with real depth. Isn't that something really special, and doesn't it just piss you off when someone says, hey it's only a movie?