What I mean is, Ethereum Foundation or not, only some small set of people will have commit access to the upstream bitcoin repository. Additionally, because of economies of scale, it makes more sense to pool resources under one central roof to solve the proof-of-work.

There might be a convoluted and informal political process involved before merging changes to the protocol. Mining companies might choose to limit their size in order to protect the network and thus their income streams. But ultimately, the number of core developers and miners, i.e., those with real power, is much, much less than the number of merchants and users.