The hard limit on BTC requires mining to eventually be funded entirely by transaction fees. It's unclear whether this will work. Is there any successful PoW cur

The hard limit on BTC requires mining to eventually be funded entirely by transaction fees. It's unclear whether this will work. Is there any successful PoW currency that runs on transaction fees alone now? How sure are we that this model pays enough to miners to keep them outspending a hypothetical attacker? For PoW to remain secure, mining honestly must always be more profitable than mining dishonestly. In other words, the total amount paid to miners over a given period (say, 2 hours) must be greater than the potential profit from a 51% attack over that same period. At present, through block rewards and fees miners are getting about $1.5 million every 2 hours. Due the the competitive nature of mining they are probably spending nearly that much in aggregate, on rent, hardware, and electricity. That's a fairly significant amount of spending to match, if you wanted to out-hash them and perform a 51% attack. However, only 1-2% of that is from fees. Almost all of it is newly minted coins. If the current level of transaction fees was all that was securing Bitcoin, we would be talking about only $30,000 of hashing spend per 2 hours and a 51% attack would look pretty darn feasible. I've heard it said that users will pay more in fees as the coinbase reward shrinks to zero, but there doesn't seem to be any reason for users to increase their fee payment to support the security of the network. Each user will spend just enough to get their transaction included in a block. As long as there is not much competition for block space, there is no need to pay more for fees, security be damned. So far in the history of BTC halvings we have seen fees rise very slightly, but not nearly enough to replace the lost block reward. Mining revenue (in $) dropped sharply following the last halving, and what restored it was not users picking up the slack in fees, but the price of BTC increasing, making the 6.25 BTC minted reward worth more. Obviously that same effect doesn't help you when the minted reward goes to zero. ---- Another way of thinking about it is that with a minted block reward, HODLers are "taxed" (in the form of dilution) to keep supporting the security of the network. In a 21M coins, no-more-minting, fees-only model HODLers expect to get their security for free, fully supported by those suckers actually transacting in Bitcoin. I'm not convinced it will work out.