So, tl;dr, there's literally a metaphysical force referred to as "fate" that exists in the 'Verse, and we human beings of modern days lack the necessary awesome to do jack about it on our own, ergo, there can be no modern Heroic Spirits without, as Emiya did, cheating.
I actually follow this. Cool; in two days you BL folks have pretty much cleared up something that's been bugging me for 3-4 years now. Score one for the decision to join!
New question: What's the cutoff for "modern"? Jack the Ripper made it into Apocrypha, but as a corrupted/villainous figure she'd presumably be subject to different rules...