Every fabric of the universe is energy. Usable energy is just establishing the ability to route the fabric of the universe into patterns of motion that suite our will. As humanity grows in sophistication, our ability to control the patterns of energy around us will be completely established and quantity of energy, in the form of electricity and heat, will be just a matter of infrastructure. Eventually the economic cost of energy will be near zero. When ASICs reach their peak mining efficiency and manufacturing of ASICs also reaches peak efficiency, one's mining power equates to available energy one has access to + cost in energy to produce the required ASICs to mine that energy. At this point, costs of bitcoin mining will be so incredibly low, that transactions fees will be reduced to negligible amounts, but the mining will continue, in absence of reward, because there will be literally no cost to it. Bitcoin mining will become simply a way to use up the free excess usable energy left over from all other energy requirements of humanity. Maybe eventually, when usable energy storage technologies are perfected, we will simply scrap crypto coins and just use pure energy as a form of currency. That seems like the next step in the sequence: barter => physical coins => digital coins => pure energy? Nothing is simpler to exchange and more universal than usable energy, so why not? And nothing has more reassurance against counterfitting than energy, due to our confidence in the law of conservation of energy. Infrastructure to transmit large amounts of energy over long distances I'm sure would be trivial by this time; so currency could disappear and the world economy could operate on the basis of exchanging energy alone.