I have explained a few times, here and on reddit, how a miner or cartel that controls the majority of the hashpower could in fact force a change in the protocol against the will of all other players.  

The procedure that the miners would use to achieve that is not complicated, but understanding it requires a bit of social intelligence: the ability to put oneself in other people's place, and think of how they would actually act in response to one's actions and changed circumstances -- rather than how one would like them to act.

Most bitcoiners just refuse to understand the procedure ("it is impossible to make someone understand something when his revenue depends on him not understanding it").  The few who do understand believe that they can counteract it by the "big red button"defense: let the whole network mine the cartel's version, while the faithful bitcoiners will throw their bitcons away, create a new premined altcoin that can be mined only with CPUS, and declare it to be "the" true bitcoin.


Yes, clients can get and recognize "the" valid blockchain as long as they can contact only one honest node; and the verification can be optimized, as you say, if one of the contacts is honest and is aware of the "false" blockchain(s) that other nodes(s) may be serving to unwary clients.

But that will not prevent the miners from imposing a protocol change, because they will do it in the open and there will never be any question about which is their "reformed" chain and which is the "orthodox" one.  In fact, clients would have to upgrade to the miner-provided software to be able to receive and interact with their branch of the fork at all; and would be forced to do so in order to regain control of their coins.