Their concerns are not entirely unfounded. We have seen, even in 5.5e, changes that indicate that sensitive topics are still something they are willing to change. Altering the pictures and text of some creatures regarding gender, comments in the past about slavery, etc. Enough change to these topics is still present to give us pause and concern over how they will hander Dark Sun, which has the topic of slavery front and center, pervasive in Dark Sun society. I don't think we will see slavery handled in the same way, or to the degree that it was present in 2e. I also don't think it will be gone. How well they handle it, though, is a mystery.
The last three settings that I've looked at are Spelljammer, Planescape, and the new Forgotten Realms stuff. On a scale of 1 to 10, Spelljammer rated a 1. It was worthless. Planescape was handled decently, so I rate it around a 6. Not as good as I had hoped, but far better than spelljammer. The new Forgotten Realms stuff was in my opinion very good, so I rate it an 8. A 9 being fantastic and a 10 being perfect, and it wasn't either of those. So we have one crappy setting, one middle of the road setting, and one good setting. That makes predicting Dark Sun very hard. The only good "pattern"(in quotes since 2 isn't enough to establish a pattern) is that the settings seem to be going in the right direction.