This would be one of the possibilities that I couldn't think of, although I'm not sure whether "Ascenson via Karmic Rebalancing" should count under deus ex machinas. While it's baseless conjecture at this point, I will concede that it's just as much a possibility as restructuring the system itself to work without witches with what we currently know (which is slightly more than zilch).
I honestly have no actual argument against this, so I can put it up alongside the other alternate interpretations that completely undermines that portion of the argument. We have no real idea of what "potential" is beyond an arbitrary narrative value (certainly you'd think Homura would've had a lot more potential given her ironclad willpower if that interpretation is true), but it's supported by just as much evidence as the Alien Ethics theory and you could measure that in teaspoons, so no real argument to be made there. Honestly I actually think it fits better thematically than Alien Ethics, but I've made my bed.
The rest of it is also pretty well-reasoned; I can't argue that they are perfectly valid. Looking back, we certainly can't argue that Kyuubey's behavior around Madoka is a special case and then make generalizations regarding its behavior around Sabrina when she is also a special case.
(I also agree with the bullshit thing, but the Doylist perspective shall not avail us here, and so we must try to fit the puzzle together with the pieces we are given. Even if it came in a blank white cardboard box, half the pieces are missing, all of the rest are perfectly square somehow, and I think one might actually be a cheez-it.

)
EDIT: Crap, I forgot to actually address
@FlatlineAskari's argument directly. Oh well; my response to Onmur basically covers what I would say in light of what Redshirt Army said. Hope you don't mind!