Yes, that seems like a good way to me. The only drawback I see is the lack of any mechanical drawback to defiling.With regard to the how-to-implement-defiling subthread: I do not think preserving or defiling should be a subclass, I think it should be a choice every time a(n arcane) spell is cast.
If I was doing it, I would make defiling equivalent to wizards in other worlds. Wizards could cast spells at -1 effective slot level (so you would need a 3rd level slot to get cast a second level spell) to have a shot at not defiling, or -2 effective slot level to guarantee it. Conversely unrepentant Defilers would lean harder into it (perhaps with a feat) to get a bonus to effective slot level. Maybe also have a countervailing Preserver feat that makes casting without defiling a little easier/better but not too much.
I think that achieves everything I set out to: Maintains the defiling-as-fossil-fuels metaphore. Makes it possible (but not too easy) to defile accidentally. Makes it a perpetually-tempting power boost, without making it inevertable for the careful. Doesn't add a die roll to every casting (just the edge cases, which is not too onerous.