Bitcoin will remain valuable in the future and there is nothing going to happen that we have already perceived right form now, even after the entire bitcoin has been mined, nothing changes from the usual that we have, it still remain a profitable asset, decentralized digital currency, miners earned rewards from transaction fees, the values increase base on the demand and supply rate as people are buying and selling, the network remains intact, nodes secured, transaction irreversible and everything remained transparent on the blockchain technology.
Costs will be high because the price of Bitcoin continues to rise, and we don't know how much Bitcoin will be worth at that time, whether it's a million dollars per Bitcoin or more.
However, these miners will undoubtedly focus on validating transactions and prioritize pricing there, both for operational costs and the profits they must earn to meet their needs and desires, just like any other human being.