This is not true. People who run full nodes (as a group) have just as much power as miners. If they all run a hard-forked client, then the miners will be happily mining away on their (worthless) chain.
The frontend has code to verify the decision of the miner, and it has no ability to change anything.
But, if all the users agree to use frontend code that rejects the decision of the miners that refuse to play along, then the chain the miners are working on is effectively dead.
The miners aren't trading any goods or services for their bitcoin. If merchants and other major players in the bitcoin ecomony go along with the hard fork, the previous chain is dead.