That's what I don't get.  If anyone could arbitrarily alter the protocol, they could rewrite it to route 10% of all transactions to themselves So can't the protocol never be altered ever without shutting down bitcoin and starting over with a new chain?  Who could alter the protocol and how?

The way I understand it, the writers of the bitcoin client software could just rewrite the code to say whatever they want and say that is the new protocol.  As long as it doesn't contradict existing blocks, it would technically "work."  But if there were 3 bitcoin clients and 1 decided to do things differently, their data would get rejected by everyone who has the other 2 and thus the network as a whole.  That's why I'm a little concerned that the entire network is in the hands of a couple people who wrote the one and only client software.  Didn't they publicly release the code too?!?!  There should be as many bitcoin clients as torrent clients at this point.  For safety reasons, someone write another one! lol.

(of course then if there were 100 clients, any one of them could be rigged to steal your wallet so if you want to use an alternative client, you have no idea if it's legit or not)