That would require that the Warp is able to affect things even when it is closed off. Otherwise, all that would result is the Culture doing a full reload from backup of every single organic in their civilization. (the workings of all the organics being fully understood in the real, their organics don't require the warp to work)
But clearly, they would distribute reload technology everywhere before implementing the plan. Or only implementing it across a section of the galaxy.
And in any case, if the pylons do cause a warp-null area, why don't people die when they walk into such a field? (I understand psykers can't do their thing, but that's it, no?)
EDIT:
(say, they claim a large section of the the Eastern fringe and just isolate that + the insides of all their ships from the Warp and leave the rest of the galaxy alone)
Actually, this is the rather more likely plan, given the Eldar won't use reload technology, and that dissociating the whole galaxy would result in a mass death like never before seen.
So let's run with this as the actual "no more warp... for us" plan.
The Culture, of course, doesn't see it that way. They don't regard the warp-part of the body as being part of the person.
But that's a cultural clash about to happen in the future. Maybe I'll hint at that idea for the Eldar part in this part 7.5. =D