Athas was cut off with very few exceptions such as Dregoths planar gate, the Ravenloft mists (which were multidimensional).
That was a later addition though - in the original boxed set, there was nothing inhibiting planar travel. The Encounters chapter says that "Fiends from the Outer Planes Appendix (MC10) can travel to and from Athas at will, but do so rarely, only when summoned by a dragon or a great wizard." You also had the Black Spine adventure, which was based around an attempted Githyanki invasion.
The planar blockage didn't become a thing until Defilers & Preservers, which was one of the last books of the line.
I could see monks meditating in the harsh wilderness of Dark Sun.
The Will & The Way, the Dark Sun-specific psionics splatbook, had a psionicist kit called the Sensei which traded some psionic power for unarmed combat skills. Monks, or some other form of martial artists, make perfect sense in Dark Sun - it's a world where good weapons are rare, and where mental discipline is encouraged in the form of psionics. That's pretty much the same kind of environment that encouraged the development of unarmed martial arts in our world.
IMO, the main reason the monk wasn't included as a core thing in Dark Sun was that it wasn't a core thing in AD&D 2nd edition. It was removed from the 2e rules for some reason.
It's less to do with the Great Wheel (that is a different thing altogether). How much of the PHB is valid? (Races, classes, equipment, or spells?) How much of the DMG? How many monsters from the Monster Manual? How much new stuff needs to be made to have it even function at a basic level? At a certain point, DS looks less like a D&D setting and more like a new game stapled onto the D&D combat system.
I hope you never looked at the Jakandor setting for 2e. That would
totally blow your mind. Humans only, limited to a certain number of kits matching the two cultures involved, limited equipment (although not as much as Dark Sun), pretty much no sapient monsters around. Removing those things lets the setting focus on the things that
do matter, mostly the conflict between the spiritual, honorable, and warlike Knorrmen and the intellectual, pragmatic, and conservative Charonti.
To me, D&D is a toolbox. Every race, class, spell, feat, monster, magic item, or piece of equipment is one of the tools in the box (it's a
box of holding). Depending on what you're building, different tools are appropriate for the job. If the thing being built doesn't need a hacksaw, I don't use a hacksaw to build it just because I have to use all my tools. Similarly, if elves don't belong in the setting, I won't feel required to put them in.