If bitcoin supply eventually get exhausted, there will always be chance to transfer within the circulating one, and this is why many people believe that bitcoin has the tendency of increase to about 1 million dollars per one when the supply becomes very scarce and miners can no longer mine again, other than purchasing from already existing holders.
Dont forget the fact too that we still have other good coins too that can serve this purpose, like Ethereum.
How on earth can bitcoin supply get exhausted? It is a currency as much a value reserve and people will continue to use it. Look at the transaction history and statistics. Blocks getting more full every day. More transactions per second every year, and this is even with Lightning Network taking more and more transactions off the chain.
If no one uses the coin, there is no demand, then there is no use for supply.