Economics is already forcing some users to recycle the heat from ASIC's as space heaters and this demand will continue to grow. Additionally, the coinbase reward for mining is slowly being replaced by Tx fees which will continue to grow rapidly especially when sidechains and inter payment channels like the lightning network are scaffolded on top of Bitcoin to really scale bitcoin up from 4-7 transactions per second to over 100k tx per second. These proposals do not depend upon PoW (proof of work... but certainly supplementing security by this mechanism helps secure the network verses pure PoS. ) and upon other means to secure bitcoin like oracles, ricardian contracts , and mutisig. Thus the amount of energy needed for PoW does not need to scale proportionally with the transaction volume and the environmental concerns are wildly exaggerated.