Though it is unlikely to happen but, my explanation is this --after the mining process is over, miners will no longer receive new bitcoins as a reward for verifying transactions. However, transaction fees will still be paid to miners to incentivize them to continue verifying transactions and maintaining the security of the network because the security of the bitcoin network will continue to rely on transaction fees incentivizing miners.
But why there are such people thinking too much ahead of time and scratching their heads about the impossible things that happen?