I think you're bound to be disappointed on this score. I agree with you btw, but I don't think severe is going be the goal. I think we'll get a very similar bit of verbiage to the one in Ravnica. I'm fine with that though, but the decision rests with the DM anyway, and mostly that's me. It would suck to finally get to play new Dark Sun and end up playing with a bunch of yahoos who want to be edgelords and play the non-genre stuff.
The trouble is that the setting flows from the restrictions.
Ravnica is part of this whole system of planes, and people do come and go, and it's also a vast, diverse city-world, where there could be an enclave of virtually any race or the like.
Dark Sun is the opposite - it's locked off from the rest of the universe (in a more severe way than Eberron, where it's kind of immaterial, because Eberron is a by-design kitchen sink), and the races being different, being post-apocalyptic versions of themselves, changed by this changed world, are a huge part of that.
If you even allow for other races in the world-design, you're inherently moving away from everything that makes Dark Sun, Dark Sun, because most of those races are designed for relatively cheery, green-and-blue fantasy worlds.
I don't think the players will really be the problem. Even edgelords - they're usually keen on the Athasian races, in my experience. But I think if the design of Dark Sun itself takes a Ravnican-style "Well, yeah, any race or subclass might exist here! Though they'd be really rare!", it just completely destroys the whole deal.
And with Dark Sun, they are necessarily going to have to make hard decisions - particularly re: Defiling/Preserving and the fact that Arcane and Divine magic aren't even really "a thing" in 5E. Defiling/Preserving will probably have to be basically "bolted on" to Arcane-designate casters - i.e. Wizards, Sorcerers, Bards (if Bards are even in DS, I suspect they will be), EKs, ATs, and so on. They'll also need to look really hard at Divine casters. Clerics can't have the god-domain-based stuff they do currently. It's just not viable.
If they're doing stuff like that, why not actually do something daring and go further and say these races and subclasses are banned by default here. I think for new 5E players, i.e. people who started with 5E (which is a lot of people), that's going to be really exciting. I don't see any reason why it wouldn't be. It was exciting back in 2E, for us. Yeah, Dark Sun banned half the books and stuff we already had. We didn't go "OH NO THEY ROBBED US!!!" or something, though, we went "WHOA DUDES EXTREEEEEEME!!!" in a '90s voice and it was awesome, and kids today will likely do the same except in a less '90s way. Just work with Beyond to add a "Dark Sun button" that DMs can flick on, which auto-eliminates non-DS-legal non-homebrew, and then online it's even easier. Pretty sure Beyond already "has the technology".
I think if you did this all really well, and also started at L3, and with generous stats and a free Psi feat, as I described, you'd get MASSIVE engagement with Dark Sun, because you have this huge generation of younger and new players who really haven't seen anything like that. It could become a whole thing. Especially with streaming and so on - the extra drama and over-the-top-ness of the setting (plus the inevitable burst of dress up in post-apocalypse-y outfits!) and even the rules (weapons breaking at vital moments etc.) would really work. I say this is the time for boldness, not trotting out some meek and slightly sad version of Dark Sun.
You gotta try something different sooner or later. Dark Sun is the exact right thing at the right time to try something different. The vast majority of AD&D settings are defacto kitchen sinks. DS isn't, and that's what makes it DS.
The one possibility I could see working without what I've said above is a "reimagined" Dark Sun. But that would need to be drastic - you'd basically have to nuBSG Dark Sun, i.e. just take the core concepts, and rework the entire setting, like from first principles. And I think you'd probably end up with something that wasn't necessarily appealing to new or old players, unless you did an amazing genius job.
