'Perfectly fine' it is not, but as with anything, it depends on your audience and how you use it. In DS it's pretty much baked into the part of the setting of the Evil city states, without those, there is imho very little left of DS. DS is an Evil setting in a different way that Ravenloft is an Evil setting, DS is more gritty and touches some problematic RL related issues. In DS all the problematic RL related issues are constantly in your face and that's why WotC is seeing this as a no-win scenario. Dark Sun has always been a niche product, so sales would always be relatively limited. And WotC already messed up with the Spelljammer reboot, even with all the bleaching of the setting, still things got past the bleachers and they had to apologize publicly for it. While many old skool Spelljammer fans just didn't bother picking up the product due to the bleaching... So DS has a bigger chance to blow up in WotC's face while potentially getting even fewer sales due to old fans not accepting the thorough bleaching of the setting. Why would WotC even bother with it?
Imho, DS could get a proper treatment if it was treated like an adult expansion, like the old 3E Book of Vile Darkness. But I wonder if certain loud groups on the Internet wouldn't still blast away at WotC because of it.
Slavery is an issue, even though it's done in an Evil empire. While from the PHB you can play an Evil Necromancer that kills the masses and turn those into it's undead minions... The big difference: we can collectively ignore that possibility. We don't talk about fightclub!
So slavery isn't 'perfectly fine' in a fantasy setting. But that doesn't mean it can't be acceptable in your fantasy setting to you, your group or even a (large) DS fanbase. But it can be to many others not acceptable. Imho it completely depends on your group. Mine generally likes to play the heroes and fighting evil. The evil doesn't need to be detailed in gory detail, nor do they need to hear what a pack of rampaging Ogres does to a village in detail. They are playing in an action movie, not a historical documentary or a gore slasher movie... But I've also played in groups where more edgy subjects are perfectly fine.
Why are both Paizo and WotC going this way? Because this way they can reach the most amount of people while appeasing certain loud groups. Both companies have a large (potential) market that appears sensitive to these issues.