*shrug* Honestly, biscuits, this isn't the right hill to die on. A better way of making your case would be to note that Stands are borderline immaterial familiars in the first place, and argue that as Servants-alike they can grasp the immaterial. So of course with a part of yourself that can see and touch a soul you could steal someone's Magic Circuits.
My answer to that argument is that even if you successfully transplanted Magic Circuits you'd have the same problem with rejection that Magic Crests have. You'd have to find some one-in-a-million literal "soul-mate" to steal from, hope they have enough Circuits to be worth stealing from, and then even then you'd be left with a very bad case of what's essentially dysmorphic disorder. Rin gets hit with a horrible sense that her arm is not hers, rip the damn thing off right now every time she uses her Crest she has to put with it; our hypothetical Stand-user would have to live with that every moment they use magecraft, if not every moment of their life.
Which, yannow, might be a price they're willing to pay, but it's still a big deal.
(On a side note, what's up with that? Part of the lie that the Three Families spread was that the reason you need a Servant is to grasp the Grail, since you can't do it yourself. But - um, Touko, and also the Einzbern, and the Atlas Academy. It seems like... well I guess there's a sucker born every minute even among the magi, and there's plenty who think they see through half the deception and not the rest.)