I'm immediately reminded of discworld where technical improvements in magical theory have gotten to the point where a spell that originally required the sacrifice of a human being can now be performed using a few ccs of mouse blood.

Hmmm, what if the practice of magic is weaker in the present of MoR because ritually sacrificing a few dozen peasants for purely experimental ends is considered in bad taste?

I can see Dumbledore BSODing over the discovery that Hogwarts is actually powered by the hearts of ten thousand orphans somewhere down in the foundations.