Rules of the system are implemented in the software validating the block, in so called "full nodes". It does not matter how much hashing power miner has - if they will produce invalid block, this block will get rejected by nodes. Miners have very limited powers: they decide which transactions are included (this is why we need many miners) and they decide in which order transactions are included. Users decide which token they hold (BTC or BTC Cash or Eth), which node software they run. Miners are low-margin workers.