None of those are cutting the warp off entirely. So yeah they aren't bad things at all.
Or that some part of everything needs the connection to the warp to live, or perhaps just to be something that could be identified as intelligent life.
Why don't they die? Well there is no canon reason. However I theorize that losing the warp is A) isn't instantly fatal or B) Pervasive enough that being in a small field actually does allow some access still. I think that being in a null field though is instinctively terrifying. You want to get out as fast as you can and at best you feel uneasy and discontent. (which coincidentally would make them an even more terrifying weapon then they already are.) People get a similar response to nulls, they are uneasy in their presence and instinctively hate and fear them.
Oh sure but the Culture doesn't believe in the soul at all while the Eldar have empirical evidence that it exists. Actually all races but Tau and Orks know that they have souls. Orks don't care and Tau haven't bothered to check.
The old canon is that it would be an extinction level event for pretty much all intelligent life and certainly for the Eldar. (Except Necrons who didn't have souls in the old canon.)
By new canon I don't think it's possible at all. Even if it was no one would dare to try.
By the new Chaos Codex book, destroying those pylons would allow the Chaos Space Marines to leave the Eye from whatever direction they want, instead of just through Cadia gate.