In most cases monopoly is bad, but with minting currency there is some sense in which it is good for a seniorage rents to exist. Imagine demand for holding USD

In most cases monopoly is bad, but with minting currency there is some sense in which it is good for a seniorage rents to exist. Imagine demand for holding USD grows by $20, a central bank can profit from the minting, e.g. if they make a $20 note for $.5 dollars. With proof of work, miners have to burn $20 in electricity in expectation (more if the electricity is stolen or subsidised) otherwise someone can profitabily enter. Even with proof of stake, an opportunity cost of $20 has to occur to the staker to prevent entry.