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Hero
I'm not sure what you're asking.
Do you mean, "How do the setting mechanics change if psionics is removed?"
I don't know, what changes if you remove Thri-Kreen or Muls or Half-Giants? What if Elves weren't massive jerks? Does it need feral Halfings? What if the setting allowed Gnomes and Orcs and Paladins?
If you're not going to keep the elements of the setting, why would you still call it Dark Sun? Rather, why would you not invent a better setting that fits the themes you'd rather explore instead of carving up Dark Sun? What do you gain by eliminating psionics other than avoiding the dog's breakfast of psionic class design for 5e? Why would you want to publish a setting with it's elements removed?
Do you mean "What thematic or narrative role does psionics fill in the game world of Dark Sun?"
The role of psionics is that it's the only righteous path. To make yourself better and use your will and effort to better the world. It represents the high fantasy way to do things right. It's no coincidence that the master of the Will and the Way in the novels was the most LG character in the series, nor was it a coincidence that all the bad guys used magic. The role of psionics is accomplishing everything that magic does without being magic.
You can't use magic for this narrative point because preservers are supposed to be inherently distrusted. Historically in the setting, preservers all eventually turn to defilement and need to be killed. That's what they do. That's why they're viewed suspiciously nearly as much as defilers. Hundreds or thousands of years ago the preservers said they'd keep themselves in check, and now the world is a dead wasteland. Even preservers are seeking an easy path to power, lying to themselves that they can control their lust for more magic, and eventually they will fall into defilement. The preserver is the guy who says that his illicit use of Xanax is under control. Remember: Defilers effectively get a 40% XP bonus, get bonus spells prepared, and cast spells at higher caster levels. That's the carrot you're dangling in front of your players. There's a reason that the game lets preservers attempt to defile as often as they want, but only gives defilers exactly one attempt to reform (and doing so is as difficult as a paladin's atonement).
Indeed, I would go so far as to say that, narratively and thematically, perservers are easier to eliminate from Dark Sun that psionics is. Narratively, a preserver PC should risk defilement every time they cast a spell. They don't because that's not a good game design for the same reason that critical hit tables are typically not good game design. However, it's in line with the setting's narrative and theme. I would even go so far as to speculate that preservers only exist because TSR was afraid that not allowing PC magic-users in the setting would destroy it's playability. They knew that lots of people didn't really care for AD&D psionics, and wouldn't print a setting without magic-users. It's the same reason clerics "return" in DL1 and why you carry around a staff of healing before that: the central conceit collapses under the weight of sustained gameplay (i.e., it's so the poor sap you made into the cleric without spells doesn't feel useless any longer than absolutely necessary).
You can't use technology to replace psionics because resources are too scarce. There's no metal, no wood, not enough food for animal goods, etc. You can't build industry on Dark Sun unless it's a slave industry. Metal is so scarce that a chipped metal knife is virtually priceless.
You can't use divinity to replace psionics, either, because there are no gods around and just elements. Clerics and druids basically get what power they can that has been offered by the elements around them. From the same world that's being killed by the defilers, templars, and sorcerer-kings. That's why most clerics and druids are lone hermits. I think clerics would be easier to eliminate than psionics, too. I think they only stayed because the game was often miserable to play without a healer (in exchange for the game often being miserable to play for the healer themself). Druids I would be less willing to part with. I think they fit well in the setting.
So, what's left? Can't use magic because that's taken. Can't use divinity because that's deprecated. Can't use technology because that's unavailable.
Do you mean, "Why does a campaign setting in a high fantasy TTRPG game need mechanics for extraordinary and supernatural abilities for the players?"
Because that's what the designers thought a high fantasy TTRPG needed. That's what was expected from D&D. If there aren't magic-users, there had best be something close to it because that's one of the reasons people play D&D and not some other game.
