Tell me do you trade the bitcoins directly with the miners only and that too every time? I guess no.
Consider this what if there no gold left on the earth to be mined? The remaining one in circulation would still continue to change hands regardless.

Similarly, when no new coins are mined the remaining coins would keep on transferring from one person to another. This eliminates any form of inflation as seen with fiat where the banks keep on printing notes till eternity.