Sure, but it's the good kind of "narrative conflict" tension.
The conflict is mostly for WotC's designers: do you include the moral conflict angle in your new version of Dark Sun or not?
I don't think of defiling as "Clearly Evil Villain Stuff", any more than the Dark Side of the Force. It's obviously evil and corrupting, but definitely falls into the "I can do this bad thing for the greater good/to save my friends" kind of power. That kind of narrative is common enough in fantasy that it borders on cliche.
What is important is to make defiling an option that can freely be opted into and out of, not something that defines a character build.
Besides, we're also talking about a game where the core rules let you raise the dead, and stick someone's soul in a jar so you can puppet their body. Wiping out some crops is really just a drop in the bucket.
Core D&D is pretty explicit that it's only interested in non-Evil characters. The idea of a Dark Side-style "temptation" isn't really a conflict that standard D&D is interested in. Raising the dead and soul jarring aren't Clearly Evil Villain Stuff, as far as the current design of D&D is concerned. You can be neutral and make zombies.
If Dark Sun continues in that vein, there's no reason to allow for defilers to be PC's. There's no neutral way to defile. Every act of defiling is canonically making the world a worse place for your own benefit. If you were never meant to become a dragon, then this is probably fine for a lot of tables.
If you are designing Dark Sun and you want that to be a valid temptation for PC's, then you enter a world where you are offering an incentive to do awful things to the millions of tables of all ages and inclinations that will potentially play Dark Sun. Not a cause for panic, but definitely a place where you'd want to be very careful, as a designer/publisher, considering the diversity of your audience.
There is absolutely nothing notable or morally upstanding in doing the only thing you could possibly do.
Edit: to be clear, you're entitled to want a simple, black and white game where being less than perfectly heroic is not an option. I may feel that turning Dark Sun into that game would be a terrible thing to do but, at the end of the day, that's all just my opinion.
Where I believe you're just objectively wrong is this notion that a game where PCs have the capacity to do wrong is in conflict with them being heroes. In my experience, the exact opposite is true.
If it would be easier, and possibly even beneficial, to accept the status quo, to be cruel or pragmatic or selfish or take shortcuts, but you consistently choose not to, then you're showing us you really are the good guy and your actions carry much more moral weight.
If that's not the game you want, that's fine, but this whole argument that giving PCs any possibility of being able to defile immediately undermines heroic play is demonstrably false.
You aren't paying attention to what I'm actually writing, and you're strawmanning some "notions" I've never actually articulated and do not hold. So I'll try to clarify once more:
What's in tension is a design consideration for WotC: Do you want a game that encourages PC's to engage with the question of: "Do I do evil for benefit here?"
Standard D&D as of this moment isn't interested in that. It's built assuming your characters won't be evil.
Dark Sun
could be interested in that, and I think it's more fun if it is, but if it is, and it is built without that assumption that standard D&D has, it has a bit of a problem: not every table is going to be asking that question in a way that's fun for the players. Encouraging characters to do Clearly Evil Villain Stuff opens the door to some really awful player experiences. You can deal with that problem in various ways, but that problem is real, and is difficult to solve for.
Of course, the new DS game could just throw defiling in as an option and pretend like it doesn't matter. Basically what 4e did, honestly. And IMXP that led to it not mattering - it failed to be an interesting moral question. So I think that would be some bad design.