In this video in Q&A form I'm going over some example of decentralised systems and on how and why many applications becomes centralised. Also trying to give my perspective on what's blockchain role in modern generation of decentralised protocols.
tl;dr
You don't need blockchain or dApps to create nice decentralised protocol. There are lots of examples for that (email, jabber, torrent ...). However cryptocurrencies might help to create better incentives for decentralised protocols to be sustainable. And you do need blockchain for cryptocurrencies.