Nope, it has lots of hurdles beyond psionics. How to do defiler magic, what to do with clerics when there are no gods, absent core races are all major hurdles. And there is a rabid fan base who will become hysterical if WotC get anything "wrong".
No! These are not problems. Ravnica had a text block explaining how Clerics work without worshiping gods. They can just copy and paste that with some revisions to get rid of any Ravnica-themed words.
Also, remember when they had the list of subclasses and what guilds they'd fit into in GGR? They could easily do something like that for defining what subclasses/classes count as defilers. Also, Ravnica has limited races. Theros has limited races. It's not really a problem. They can just list the races that exist in that world, the changes to them, and the additional races that are Dark Sun specific, like they did in Eberron: Rising from the Last War. None of those are problems. They've gotten similar problems right before, so why don't you think they can do it with Dark Sun?
"Easy" is a clean new setting. Does MtG have a post-apocalypse world by any chance?
You think it's easier to make a whole new setting than translate an older setting to 5e? No. It would be way easier to adapt Dark Sun to 5e than to create a new setting. Also, no, MtG doesn't have any viable post-apocalyptic worlds that could replace Dark Sun. The closest ones are New Phyrexia, which straight up would not work as a setting, most likely, and certainly wouldn't replace Dark Sun. Amonkhet could possibly work, but the only thing it has in common are that the Gods are dead, and the world's a desert.
And the polls suggest that, whilst Dark Sun is quite popular, Planescape is far more so.
Did you read my first post on this thread? (Which was also my first post ever on enworld, iirc). I said that I am expecting Planescape to come out this year, probably delayed due the pandemic. Planescape is easier to translate to 5e, because you just need to develop information on the planes of existence. Dark Sun is harder, you need psionics. 100%, Planescape is coming before Dark Sun, just because it's easier.
If they had better ideas they would have put them out first. WotC are not sitting on a bunch of alternative psionics. But even if they did then it's at least 3 months to put it through the UA process, and a 3 month delay, with no guarantee of success after that time, is very expensive for a project that is already underway.
If they are currently making a Dark Sun book, I don't think they'd be stupid with how they time the UA. They've been on this path before with the Artificer, the Xanathar's subclasses, and most of their UA content that made it into books. They know what they're doing, so they'd schedule around it. Sure, delays can happen, and are likely due to the pandemic, but they are necessary if they want to get Psionics to a point of community acceptance, which they are trying to do. It is obvious that they're trying to do this, because they've had 2 surveys on it and have revised it. They wouldn't ask for community feedback if they didn't want it.