How can they not dictate monetary policy? They could conceivably cause inflation by increasing mining rewards or perhaps deflation through coin destruction.
Miners don't decide what software other people run, that's why. If a miner tries to mint themselves more Bitcoin than they are due, then their block will be rejected by the network.
It's other way around. Majority of miners were against SegWit and blocked its activation for over a year. Community then decided to collectively upgrade their nodes to accept only certain transactions. It is called User Activated Soft Fork (#UASF)
The miners had to make a decision. Mine coins that rest of the world dont want, or activate segwit.
