I keep hearing this, but really, how? The only useful thing on this front to fall out of the blockchain is Namecoin and, despite being a really good idea, nobody knows how to stop name hoarding. There will also never be a mechanism for name dispute resolution... and who wants to register a name that could cost $5 this year and $1000 to renew next?
Bitcoin itself has and likely will continue to trend toward centralization because of the tendency for people want want lightweight clients and instant transactions. Services like Bitpay already consolidate merchant interaction, and too many people already accept the Gmailification of wallets by holding web wallets. This is probably for the best because people can't be trusted to guard their own keys. It's also debatable whether total blockchain decentralization is a good thing, given that currently the hash power is all held by a bunch of very dodgy unknown Chinese ASIC mining shops instead of a bunch of very dodgy but well-known multinational banks.
The issue with the entire post is that you can have both. I can use Facebook messaging to talk to my family while using IRC to answer questions about Java or talk to strangers about video games. They are not mutually exclusive. Just because there are these centralized, mega-services does not mean that the decentralized ones stop existing.
Diminishing the active population of a service may result in less software development targeting it - resulting in a death spiral to the profit of the centralized services who reached critical mass.
Even when the open protocol becomes dominant through network effect, when a major service provider attracts a significant population fraction he can fork into a centralized proprietary direction : see for example how Google used XMPP to strengthen its initial Google Talk user base and then cut them off from the rest of the XMPP world... Embrace, extend, extinguish.
I'm still using IRC - but it is an IT ghetto. I'm still using XMPP - but only with technophile friends, my family or other people to whom I am a prescriptor. This decline may be caused by chat no longer being considered a standalone functionality but an adjunct to web applications that implement a social network...
Another benefit of centralization is efficiency - when everything is distributed, everyone does all the same work over and over again, which could be done just once.
The question is, how can we keep centralized just the things that (users of them) really benefit from it, and let the rest be decentralized?