There's no need for everybody to serve the entire index. Set up a DHT and let interested parties exchange the NZBs. All problems in computer science can be solved by another level of indirection.
DebTorrent was to be a set of BitTorrent extensions to allow the Debian archive to be syndicated as one ever-growing torrent. Seems similar to the use case here, and free from the problem we have now which is the huge indexes getting hit by takedown notices.
http://wiki.debian.org/DebTorrent
it's a reputational and data quality issue as well. completely decentralized content distribution doesn't usually work unless there is some kind of curation, there are just too many motivations for disruption. it's not an impossible problem, but long ago ceased being mostly about the technical occupation of shipping bits between nodes.
You could always have sites that only serve up indexes of NZB files (e.g. "NZB file => SHA-256"). Then you could use these to determine which NZB files to trust. I would assume that distributing a "NZB => Checksum" hash would be a lot smaller/easier than running an entire site on top of a terabyte of NZB files.
