Miners are not able to adjust the monetary policy of bitcoin, because attempting to do so causes a hard-fork, which would have them mining a chain which has no economic use in the real world.

The chain which everybody is using in the real world is the only one that matters. Miners are forced to mine blocks which adhere to the consensus of economic users. If some people run a million "Sybil" nodes, it makes no difference, because those sybils do not have any economic use, they're effectively ignored.

Bitcoin is not a democracy. There is a network effect which is now far too large and diverse for any small group to sway, and with time, the protocol will become further ossified to the point where it will literally be impossible to enforce changes, because there is no way to coordinate them globally to so many users.

The only control a majority of miners can have is that they can withhold transactions from the chain by orphaning any block they don't want to include. That is, they can cause denial-of-service for as long as they hold the majority of hash-power, but in doing so, they withhold revenue from themselves by failing to include paying transactions into blocks.