Not much energy is required for a transaction, but the network of legitimate miners has to have more hash power than any attacker. Once the block reward gets ti

Not much energy is required for a transaction, but the network of legitimate miners has to have more hash power than any attacker. Once the block reward gets tiny, the profit will come from transaction fees. So it will depend on whether the users collectively get enough benefit to pay the miners. If the attackers get more hash power, the blockchain will lose legitimacy as the source of truth and the whole thing will collapse.