Why the set of developers is considered centralised? I can write an implementation according to the specification and have no relationship with developers of another implementation.
My understanding is that the specification changes over time, and if your implementation doesn't match the central one, then the rest of the network stops interacting with you. That is: there is a massive network effect of people using github.com/bitcoin/bitcoin as canon, and there is a central group of devs with write-access there. While you can fork it, unless the rest of the network gets in on it, you've just created some altcoin that nobody cares about.
Maybe you can, but have you? Has anyone? Are there any nodes on the bitcoin network not running the same code written by the same handful of people?
Also, saying that "all you have to do to participate in this new trustless economy is to read the spec and implement your own validation node and client, and maintain it as the protocol changes" is a pretty different promise than "just download this app".