* Usenet was a decentralized forum, like a decentralized HN in a sense. The first attack on it were the small-time commercial spammers. Then the government, RIAA/MPAA and last mile net duopoly (AT&T/Verizon) conspired to more-or-less sink it, which they did ( https://www.verizon.com/Support/Residential/internet/highspe... http://www.tomsguide.com/us/AT-T-Usenet-Access-Closed,news-4... http://www.tomsguide.com/us/Newzbin-Usenet-MPA-Copyright-Inf... )
* Newer peer-to-peer file sharing apps never had a chance. Napster, Gnutella and so forth, under attack by the RIAA/MPAA and government.
The trend is toward more centralization, not centralization. With a last-mile duopoly, antagonistic government and MPAA and RIAA beating war drums, decentralization has been under attack for years and the centralization forces have been winning. I see nothing in the visible horizon that sees things changing any time soon.