That's a common misconception. The miners can do what they like, but if they start mining blocks that the exchanges (and everyone else) don't accept the blocks

That's a common misconception. The miners can do what they like, but if they start mining blocks that the exchanges (and everyone else) don't accept the blocks they mine, then they can't sell their coins. Bitcoins democracy comes not from what the miners think, but what the people who accept bitcoin as payment think.