Ignoring for DM fiat, there is no rule in D&D for losing your class powers. Not for clerics, druids, paladinr or warlocks. It's a fine house-rule, and it's kinda implied sorta-in Oathbreaker paladins, but RAW there is no mechanic in play that takes away your spellcasting.
Which is part of the problem with this exercise; we're still applying definitions to spellcasting that no longer is universally applicable. When Dark Sun was created, there were only two Spellcaster groups: Priest and Wizard so it was easy to lump everything arcane into the Wizard group and everything Divine under Priest. Likewise, 3e precisely defined Arcane and Divine as having innate qualities (like Arcane Spell Failure or Divine Focus components) and 4e lived and died by power-source and role. But 5e has no such structures and even less consistency.
Dark Sun is going to have to force that structure onto 5e, and it's not going to be an easy fit. It will either have to do that class by class (and ignore areas where power-source changes, like the aberrant mind or divine soul) or do so by subclass and be outdated the minute a new book with subclasses in it is released. Neither is as clean as the older editions, but that's the price we paid for having 14 classes try to do the job of 30+ from editions past...