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 already a consensus (pun intended) around Crypto that BTC has am expiration date but has opened the door to the mainstream world for other Cryptos. When that point comes then you are right in saying that miners will be paid in transaction fees but that point is basically a point not even giving attention to because by that time we will all have moved away from a coin like BTC because the world will have adopted a coin that has moved to more sustainable form of Proof of Stake such as ETH, which seems to be the closer contender in the pool of Crypto. We are all aware of the amounts of energy consumption within Proof Of Work so there are greener projects such as Bitcoin Origin being discussed which would use Proof Of Ambassador which looks to be much more sustainable and scalable for real world application. Savvy investors out there seem to be leaning towards greener projects as potential investments so watch for that coming wave