I'm not sure I understand your point. Yes, in order for a Bitcoin fork to succeed, you'd have to convince the majority of clients to use the new protocol. However, once you gained a majority, the Bitcoins generated by the old protocol would fall in value relative to the Bitcoins generated with the new protocol. If your Bitcoins are worth $100 using the new protocol, but only $50 using the old protocol, there's a strong financial incentive to start using the new client so you can sell your bitcoins at a higher rate and to a larger audience.
Yes, it's centralised (to a degree), but when people talk about decentralised protocols, they usually mean that the protocol itself is decentralised. There's still usually an official or de-facto standard maintained by a relatively small group of people.
Your argument that Bitcoin is not decentralised could easily be applied to any P2P protocol. I don't think there's any P2P system around, except perhaps natural language, which has a decentralised specification.
