Once all bitcoin are mined, then miners would still receive rewards but not as much as they do now. Their rewards would be restricted to only the transfer fees generated in the network. It will take a long time until all bitcoin are mined. So, it is reasonable to assume that by then, the bitcoin developers will come up with something to solve this. Also, if bitcoin is adopted by 10x or more people in the world than, number of transfers will increase. This will generate a lot of fees as rewards for miners. Another choice for bitcoin developers would be POS or proof of stack. This will surely solve this problem without any doubt. But it will take a long time to develop. So, I don't think we need to worry about that now.