I think an important thing to keep in mind is the bestiary of Dark SUn has a lot of psionic creatures (mutants, tribes, and beasts) that really spice up the setting. Dark Sun is all about constant survival warfare, and creatures have evolved a way to deal with that: psionics. There are psionic cats, psionic tigers, psionic grasses (that are murderrous), psionic catci, psionic insects, even psionic ghosts. If you take out Psionics, Dark Sun loses its weirdness in the bestiary and with its monsters - which is a big shame.
This is a decent point -- psionics fills the weird trope, which is present in Dark Sun. I don't think it's at all necessary for the dangerous or survival themes, and I think there's a lot of room to have lots of weird without psionics, but it's a solid point.
Overall though I find the conceit of this thread just pretty thickheaded. Its like saying "You can have Forgotten Realms without magic." like huh ??
This, however, is uncharitable. Dark Sun, as a setting, evolved largely without psionics, according to the designer interview I link above. It was towards the end when they agreed to incorporate psionics because it was in development and it went with the weird themes. If psionics had not been in development as a separate thing, we very likely would have had a Dark Sun without psionics. Instead, the designers worked to incorporate it. However, the designer interviewed did say they didn't like how psionics was implemented in the Psionics Handbooks and how that worked with the system and would have preferred a reworked psionics that only PCs and major NPCs had. This, again, cuts against your statement here -- we almost had Dark Sun with a lot less psionics, perhaps even none, but for a quirk of conincidence.
I get the argument that this is how it was written. My question isn't to examine what a Dark Sun without psionics looks like, but rather to ask if the psionics included are necessary for the themes of the game. I don't think they are, because psionics was an add-on later in the development cycle, and the designer himself felt the game would have been better served with less psionics. That says to me that the major themes of Dark Sun are largely independent of psionics, which is interesting because that would mean that psionics as a system can be very different and still operate in Dark Sun. We saw this in 4e.
As for Forgotten Realms without magic, far too many of the core concepts of Forgotten Realms are rooted in magic -- it's not just a layered on system, it's fundamental to the themes and tropes of the game. So, no, if you would care to actually ask if Forgotten Realms needs magic, I have some points on why I think it does, foundationally, and not because there's a number of magical monsters but things that go directly to the foundational tropes of the setting.
