Xilizhra wrote...
That might be relevant if any scientific studies had been made of conditions in Arlathan, but none at all have. It's like looking at pure lead and using it to study the radioactive materials that decayed into it.
Arlathan was in Thedas, it was destroyed by tevinter and inhabited by elves.
Thedas is still around, Tevinter is still around and the same goes for the elves. And after studying all three, we can conclude that there is nothing in physical Thedas gifted with eternal life nor that humans cause magical disease in other races.
Therefore, there is nothing supporting the elven legends.
Dalish history suggests that there was something. That's just one hypothesis. There are others, such as the immortality being granted by something in Arlathan that was lost when the city was destroyed, or the immortality being an indirect loss of the universal magic the elves once had.
"The elves said it so, it must have been something."
You lack evidence. Evidence is what distinguishes theories from fanfiction. Do you actually have any evidence that humans carry some plague, that there was "something" In Arlathan, that universal magic actually existed?
No, because there is none. If you don't find evidence that supports a claim and do, in fact, find evidence that goes against it, you conclude the claim is wrong.