I don't think that you "get it" - if we have multiple versions of Bitcoin then what are the exchanges supposed to do (they can only accept one if they don't want to lose money)?
So your idea is to decentralise something that basically cannot be (have you really thought this idea through?).
Forking is not the same as creating alts - so if Gavin and others want to create an alt then why don't they just do that?
You are correct , insomuch as there cannot be multiple competing implementations that break consensus rules. There can cooperatively be multiple implementations that have different developers (as long as they coordinate with other implementations), and written in different code , different languages, and with different features that don't break consensus. This all ads a great amount of complexity and coordination effort, but it can and should be done for security and diversity of choice.
For hostile implementations attempting to break consensus rules , we should be wary of but not discourage their right to stage a takeover... as long as 95% consensus is met in the economic community we should even encourage this. If 95% of the economic majority decides on something dangerous or against bitcoins foundational principles , than so be it ,.... we should allow the project to run its course, and those who do care about something truly unique and different than paypal 2.0 can leave and create our own alt.
I certainly will sell my bitcoins and leave the project if it becomes centralized and controlled , regardless if its done By Blockstream or Coinbase and the blockchain alliance or breaks any of the foundational principles that make bitcoin unique (and lest more misleading statements are made , raising the block limit in a reasonable fashion is fine).