These are good questions -- does the setting change? I'd argue changing races is probably not much of a big deal, so long as replacements are suitably post-apoc. The themes shown by the races aren't specific to the races, but more that they changed existing races to fit a post-apoc theme and outlawed a few races that they felt didn't fit. There's no reason you can't fit them in, and no reason you should. Still, no one I know says things like "WotC needs to come up with a better Mul for Dark Sun!" They say that for psionics. My question is -- what does psionics actually do for the themes of Dark Sun outside of just being psionics?
Yeah, and I'm saying that question is nonsense on it's face. It's like asking, "How do you define a noun without referencing an object?" You don't because nouns are objects.
"What does magic do for the themes in Greyhawk outside of just being magic?" What it does is be magic. It's not mundane. It's job is to be fantastic.
"What does psionics do for the themes in Dark Sun outside of just being psionics?" What it does is be psionics. It's not mundane and not magic. It's job is to be fantastic and not magic.
That's really as complicated as it needs to be.
Slippery slope arguments aren't terribly convincing. Sure, at some point if you replace enough stuff you end up asking Ship of Theseus questions. I'm not asking people to defend everything, or suggesting nothing matters, I'm asking specifically why psionics matters. This argument is just chaff that doesn't address why psionics is important.
That's not a slippery slope argument. At best, it's a composition fallacy. And it's not mine. It's yours.
Your whole question boils down to, "Why is this element of Dark Sun essential to Dark Sun?" My question is, "Why wouldn't it be?" I'm asking your own question back to you and you're saying it's a fallacious argument? By asking the question, you're asserting that psionics is not essential to Dark Sun. You have to entertain that notion to even ask the question. Now you're complaining that I'm asking you to defend the assertion buried in your own question. Sorry, it's your implication. Just because it's a premise hidden by a cleverly phrased question doesn't mean you don't have to defend it.
You need a purpose behind making a change. If you don't want to make a change, then what are you asking for? There's no discussion in that case. If you do want to make a change, then state your reasoning. You must have some or else the question wouldn't present itself in the first place. You want people to defend the counter argument without needing to state your argument. If you're not going to answer why you want to make the change, then why would I keep discussing? If I challenge your unstated assertions you say that you don't have to defend them. Or, that that's not what you're saying... but you still won't say what you are saying. You just repeat the same question over and over, ad nauseam.
So, what's your point in even asking?
This is a reasonable argument, thanks. However, I think it starts too strong by saying that psionics is the only righteous path. This ignores that everyone uses it, and, in Athas, most of those people/things are using psionics for evil or selfish reasons.
Any tool can be used for evil. A sword can defend as well as betray. Athasian magic is unique in that the tool itself is evil. It's a deal with a devil. If you want to do something supernatural, and in a high fantasy RPG where you're going to be facing Templar and Sorcerer-Kings you probably do, you need to offer a solution. If you want a tool to fight against god-like magicians and that can't be magic... well, what do you use?
Why not use psionics? That already exists. It's been around almost as long as magic, just not as signficant.
And, there's lots of other righteous paths available that don't require psionics. So, to boil this down to nuts and bolts, what you're saying is that psionics is the only way to use magic that isn't tainted by defiling. Sure, no argument, I pointed that out above in my last response to you that it's a pretty decent argument that some kind of magic system is important for a D&D game and, since DS has so strongly tainted arcane magic and nearly eliminated divine magic, that psionics is the answer. I can follow that. The moral arguments your making, though, don't hold much water without the argument that some form of non-evil magic has to exist. I'm not sure, though, that the latter is really a true statement.
No, that's a nihilistic argument. That's what's bothering you about it. You don't need anything in any setting by this logic.
Nothing has to exist in any setting. The point is that, with psionics in Dark Sun, it does exist and there's a point to it being there in the form that it is. It doesn't need to be psionics to make the point it's making, but that doesn't mean it has to defend itself for being psionics, either.
I also think that Dark Sun is absolutely NOT High Fantasy. It's tropes do not align with high fantasy. Still, if you're arguing from the position that it is high fantasy, and therefore needs a non-evil magic system, that's, by far, the best argument I've see for psionics in Dark Sun yet. Kudos.
I would still qualify Dark Sun as a setting as high fantasy. It's dying earth and that warps it pretty heavily, but it's still high fantasy.
1. It takes place on Athas, not Earth. Classically, this alone is actually enough to qualify the setting as high fantasy in literary terms.
2. It's focused on good vs evil and morality. Perhaps more explicitly than any other setting. The Sorcerer-Kings, Rajaat, defiling magic, slavery, templars, etc. represent very classic high fantasy explicitly evil foes. Maybe it fits more in gray-vs-black because of how bleak it is, but you still always have a choice of doing something explicitly evil or doing what you have to. Good is kind of not good because life is just that hard, but the evil is really really evil. You have to choose between letting some innocent people die, or selling entire tribes of free people into brutal chattel slavery.
3. Victory against evil is almost never through force of arms (otherwise it would be heroic fantasy). Because you can't fight evil directly on Athas. At the end of the day The Dragon is too strong. Heck, the Sorcerer-Kings are too strong. The Templars when they work together are too strong.
4. The scale of the adventure is almost always the end of the world. Or, rather, the end of the world being even sooner rather than somewhat later. Still, the PCs are typically saving the world, such as it is. At least in the adventures that I remember. It might just be the destruction of Tyr, but when Tyr is the last Shining City on the Hill in a world of dying embers, that means a lot. This isn't Greyhawk where there's hope for other nations to endure if the city falls, or for life to endure if civilization collapses.