Yes, centralised. That's the model that works for software development. Where would this voting nonsense end? The alternative is to make development "equal opportunities", anyone and everyone gets a turn at designing and patching! Have fun with the coin you end up with using that development model 
No, you're free to operate and mine on whatever client you want. If you're forked off the network, it's your fault.
Not the user decision. That's not centralised. I didn't say that. I was talking about the development process. Not mining. It contributes to meaningful discussion if you read what you're replying to.
I'm interpreting your statement in a slightly different manner than you may have intended. By modifying my own software, I am contributing to the overall development of the collection of software capable of processing, relaying, or mining transactions, assuming I'm also making it available. Additionally, by sharing it, I'm making it possible for anyone to pick up on those changes and use them on their own copy, or integrate them into a centrally-developed program.
On the other hand, nearly all the software that is USED is developed in a centralized fashion, but that doesn't apply to the overall process itself.
I see. Apologies for being terse, but your reply sounded like you were talking about a whole other topic. Call it BIP burn.
That's an interesting expansion on the way development works in practice, and I recognise what you're talking about from browsing around on Github. There are frequently forked projects with tiny differences, and it's totally plausible that the differences in these pet projects could end up ported to the main project (or that the tiny fork usurps the original). The more access there is to the code we use everyday, and the more we depend on that, I wonder whether this might become a trend.
There will always be a small set of people or an individual with sole commit access, necessarily so. But as you say, if someone forks that project and finds a significant enough userbase, the status of the previous dev team is being manifestly questioned. Overall, it's a good thing. I think we're all aware that it can be abused as well, particularly with consensus critical p2p networks.