Anyway! One of the common complaints we make here on XKCD Sucks is that "this joke has been done before." And one of the common criticisms is that every joke has been done before, and that therefore originality is completely meaningless!
I, too, once thought as you did.
Because it's true, in a sense. Being Completely Original is certainly not sufficient to make a good story. Writing a story which has never been done before is a pipe dream at best. At worst, it's a recipe for something really terrible, where various rules and conventions are broken just because they are rules and conventions that other people follow.
But clearly originality does matter. Surely you have complained about Hollywood's tendency to just remake old films these days--a trend of excessive unoriginality. Nobody wants to just watch something which is a rip off of something else.
There's a saying, the origin of which I have forgotten and am too lazy to Google: "Bad poets borrow. Good poets steal." If you are writing a story, and that story is just Star Wars, you're not writing something anyone wants to watch. That's just borrowing the premise--it's not really yours. Stealing implies making it your own. You can write a story which is inspired by Star Wars--rebels against an oppressive galactic empire, a hero's quest, the works--and make it your own, and tell it in a way that only you could--even if the story itself has been done--and that would be worth reading or watching or whatever.
"But Rob, you fat, objectively undesirable fuck who is incapable of writing a coherent argument and is really dumb and apparently sanctimonious despite the fact that that's not what the word means, who is also a terrible writer and has the emotional maturity of an eight year old and is incapable of thinking about anything with anything remotely resembling objectivity and bashes literally everyone he disagrees with with nothing but hateful disdain, and also has no life at all apart from hanging out on XKCD sucks and is hated by all the commenters and has no friends and never will and doesn't even know what a vagina is, much less what one looks like, and would probably love the cock if he weren't universally undesirable to men and women alike," I hear you say. "What does this have to do with XKCD?"
Basically it's this: whenever XKCD does something which has been really obviously done before, it isn't adding anything or changing anything. It doesn't add to the conversation. It doesn't improve upon or otherwise alter the original. It doesn't offer an original take. It is essentially nothing short of taking an old joke, and then telling it again. There's nothing original or interesting about it.
It's very possible to lift the story from something else and make it your own, and make it excellent. Some of my favorite stories are essentially just rethinking old myths and classic literature (eg Rosencrantz and Guildenstern are Dead), and "reinterpretations of classics" is one of my favorite genres to write. Is it Completely Original? No. Does it add to the conversation and make it worthwhile? Certainly. If it didn't, it would be worthwhile to just say "but this is just Hamlet" or whatever.
UPDATE: Right after I posted this, a video popped up in my RSS feed. It is "inspired by" Matt and Kim's Lessons Learned video, which you should watch first. Then watch this one. The timing was too perfect to pass up! Is this original or is it a rip off?
FURTHER UPDATE: I googled the quote at the prodding of one of our friendly neighborhood cuddlefish. Apparently it is by TS Eliot and it goes like this:
"One of the surest of tests is the way in which a poet borrows. Immature poets imitate; mature poets steal; bad poets deface what they take, and good poets make it into something better, or at least something different. The good poet welds his theft into a whole of feeling which is unique, utterly different from that from which it was torn; the bad poet throws it into something which has no cohesion."