It's actually a bit personal so I'd rather not get into it. But basically there are two things that I view as too essential to who I am (Memories and emotions) and the unwilling modification of those two things are what I feel are some of the greatest crimes any being could do. Going through said modification would be one of the most horrifying things to experience.
Anyways like we've said before closing off the Warp would pretty much wipe sentient life out in the galaxy. Even the Newcrons would be affected. Basically the warp and intelligent life are irrevocably linked and one cannot exist without the other. Only nulls are an exception to this rule and they are very rare and almost unique to humanity as far as I know.
On a side note didn't we decide that every time a Culture member changes body (or gets reloaded from Back up) they are basically getting a new soul to match? To put it another way they died and a new being was reborn with the same memories and abilities. The Eldar may actually be able to pick up on that and it's why they would find back ups so horrifying.