"Homesteading the Noosphere" by Eric Raymond covers exactly this topic. He compares the open source software ownership to Lockean land titles. With Bitcoin, it's interesting since the project itself is a system of value and transactions.
Would they need to start a rival block chain? Or do they just need 51% of the current community to run their clients to take over the current block chain? (as suggested by a comment above)
Technically speaking, getting 51% of the community to switch is still creating a rival block chain. You don't even need 51%. As was the case in this incident, if you have incompatible nodes, the block chain would fork itself naturally.
As far as I can tell this sort of centralisation of influence is a common emergent property in most anarchic human systems.
Luckily if something particularly egregious occurs in this case, the community can easily fork the codebase and start a rival blockchain.