Why do they have to produce material accessible for everyone?
Because they're a company. It's stupid of them to not produce material that is as accessible to as many people as possible.
And what does that mean? Is that $375 mimic made for every D&D customer?
Sure. Does everyone want it, or can everyone afford it? No. But it's made
for everyone. Unless it has small parts that are choking hazards, in which case it's made for everyone over the age of 3.
I think not. Dungeons & Dragons vs. Rick and Morty was NOT made for every D&D customer, nor was Acquisitions Incorporated or The Stranger Things starter box. Nor does the D&D starter set come with braille dice...
AFAIK, none of those things you're talking about revolved around a specific type of event that not only harmed a lot of people in the past, but still continues to harm a lot of people today.
And while maybe the starter set didn't come with braille dice, braille dice do exist.
In what way can DS do harm to real people? If there are subjects in there you don't want to deal with, don't play it! Uncomfortable with HoL, don't play it! Or do the subjects in DS make people racist, etc.? If that's how that thought process goes... Euhhhhh... What would video games then do to people?
I'm pretty sure nobody has said Dark Sun makes people racist. If you believe that's what people are saying, then you
really need to step back and re-read the arguments.
But why is the inclusion of slavery
so important to you that Dark Sun can't be made without it?
I would say that murder is an
unnecessary evil, murderhobo is an actual term. And sure we can say, those were all Evil by Nature beings, that's keeping it simple!

Unless you just murdered a bunch of Lizardfolk, whom are Neutral... Let's do away with the ability to play an Evil character. And let's not forget all the tools in D&D to do really bad things, like Charm Person. We can go down an extremely deep rabbit hole, especially with D&D!
So, imagine if D&D hadn't built around the idea of going forth and heroically slaying the Always Evil beings that were performing Always Evil acts across the countryside, but instead had been built around the idea of sneaking into villages of peaceful, non-evil beings and slitting their throats, poisoning their wells, or carrying away their maidens and youths to rape and torture as you desire.
See, going around being heroic, which sometimes involves killing intelligent beings, isn't an unnecessary evil. It may not be the most optimal course of action, but it's not an unnecessary evil.
Murdering people in their beds, torturing them, etc.,
is an unnecessary evil. And so is slavery.
And here's the funny thing. Those really bad spells like
charm person have, in fact, been toned down. It used to last for much longer (on average, three weeks) and didn't let the target know they've been charmed.