They can't scale to replace fiat, but they can scale a lot more with off-chain (so called 2nd layer) solutions.
Not all PoW based coins have rewards going to zero; some have so called tail rewards and others even have constant reward like 1 coin per second forever.
Not all PoW based coins have rewards going to zero; some have so called tail rewards and others even have constant reward like 1 coin per second forever.