Comment by WJW

For Proof-of-Work coins, the miners need significant amounts of electricity. Electricity is not free, so maintaining the network costs a significant amount of money. This money is "reimbursed" to them through mining rewards, but since electricity companies typically can't be paid in cryptocurrencies the miners will need to sell (a part of) their mining reward to pay the power bill. This means that there is always a money outflow proportional to the hashrate, which somehow has to be made up from money inflows from users.

A cryptocurrency without users putting in "new" money will slowly bleed out through electricity costs. This will become even more "fun" in the future as all coins will eventually be mined and the ginormous electricity bill will need to be paid through transaction fees alone. This is one of the main reasons Proof-of-Stake is getting so much research btw, since it should use way less electricity.

(The above is true for most currencies btw, even dollars and euros bleed out money because they have to pay mints and central bankers. The difference with those is that there will always be demand for (say) dollars because US citizens MUST pay their taxes in dollars. If they don't, a number of measures up to and including prison can be taken against them. Bitcoin has no such backstop since nobody ever NEEDS a bitcoin to pay off someone. Ransomware is a rare exception)