In theory transaction fees will be what solely rewards the miners. In reality though, unless computers become smarter, faster and smaller this wont be profitable most likely. But I'm sure by then that another currency with a more Eco-friendly consensus, and more profitable rewards will surface. We should start looking to more sustainable Cryptos. There are many coins that are potentially "better" than bitcoin.
There is a new consensus protocol that is Eco-friendly called Proof of Ambassador introducted by Bitcoin Origin. Bitcoin Origin's Proof of Ambaasador consensus is based on pre-approved Authoritative validators with a Proof of Authority approach. Ambassadors are voted for monthly by the Bitcoin Origin coin holders. By staking Bitcoin Origin tokens the community is given the power to vote for the Ambassador who has represented Bitcoin Origin in the effective manner. Bitcoin Origin is a self- funded project it's not an ICO or an Altcoin yet it is an upcoming hard fork of the 5 biggest PoW Cryptocurrencies Bitcoin, Bitcoin Cash, Ethereum, Litecoin and Dash. Bitcoin Origin has Eco- Friendly approach to mining using severs that Eco- friendly farms.