What does Dark Sun do in this situation? Nothing. Evil is more powerful but it doesn't bring with it a corruption or a price. Oh it ruins the environment? Environment is already irrevocably ruined, who cares if I damage it further? What is stopping me from from defiling? Morality? Dying with moral superiority is still death. People hating me? They will respect me when I can rip their souls out of their bodies. Why would I ever choose to preserve when there is no consequence for defiling?
I’d push back a little on this idea that defiling was written as consequences-free. The problem was that mechanically the consequences were that a significant chunk of land around you was sucked dry and the earth was basically salted, in a radius depending on the spell level, the fertility of the local land, etc, etc. But it didn’t directly hurt you and so it was up to the DM to enforce roleplaying consequences for that. Roleplaying penalties to balance mechanical advantages are always a questionable design strategy and outcomes vary widely from table to table.
If you’re a defiler, you can’t hide what you are unless you don’t spellcast. If you use magic, you leave a trail of obvious destruction behind. If you do this in a city, then the plants you destroy are the sorcerer-king’s property, the crops that feed his populace. Sorcerer-kings take destroying their city’s food source SERIOUSLY. Theres a reason uncontrolled defiling is illegal in the city states. If you’re in a small free village, then every time you defile you destroy the locals’ livelihood, and they’re probably living hand to mouth as it is. They’re going to do their best to murder you before you starve them. And yeah, if you’re powerful enough you probably have nothing to fear, but if you’re a low-level 2e wizard with d4 hit points and 1 spell (or even a low-level 2e Dark Sun wizard with 3d4 hit points) then an angry mob is a very real threat. And you have to sleep sometime. And every druid in the setting will do their best to kill you on sight. As will most of the preservers because killing you early is better than waiting til you’re too powerful. And many of the other defilers because vegetation is a limited resource and if you destroy it to fuel your power then they can’t destroy it to fuel theirs.
That’s what life is like as a defiler. You can’t hide what you are, and once what you are is recognised, every hand is against you. You can’t settle down because you slowly destroy any place or any community where you stay. You become a rootless nomad, a desolation plague on legs, hiding when you can and leaving no prisoners when you can’t, forever leaving a trail of ruin behind which serves as a trail that the people hunting you can follow. You eat the world in a desperate quest for enough power to protect yourself from the very enemies you’ve made by the mere process of amassing that power.
The downside to being a defiler was … having to be a defiler. But doing the maths for how much plant life you annihilated every time you cast a spell was a pain, and the responsibility for enforcing the in-world logical consequences for defiling fell to the individual DM with obviously variable results.
However - that was then, this is now. The defiling sorceror in the UA causes no damage to plants, and only has a negative effect on other individual creatures that they specifically choose to target. I mean, the 3rd level ability even has them expending their OWN hit dice to empower their spells, rather than sucking lifeforce from outside. There very much is no downside to being THIS kind of defiler. Hell, if this is how defiling works Athas would still be green, because defiling doesn’t damage plants any more. I know the 5e design philosophy is absolutely allergic to PC abilities having downsides, and very parsimonious about allowing magic to have lasting or permanent effects beyond the next long rest, but having Athas be a wasteland because of too much defiling, and then have defilers not be inherently destructive seems to miss the point by so far you might as well be in the elemental plane of perfect spheres. I’m still hoping there’s generic defiling rules that apply to arcane casters across the board and that this subclass specifically represents those defilers who’ve chosen to specialise in and explore the nuances and advanced techniques of defiling, but time will tell.