If the same variables we have today are intact by the time all 21mil BTC mined, then BTC foundation/ community/trading partners must hire or pay miners to confirm transactions to insure continuity. Having said that I am not sure 4 or 5 years from now those variables will be there at all. Change and unpredictability is the name of the game 
If you want a transaction authorized in 10 minutes, transaction fees are already here.
I suspect that after all the coins are mined there will be a QOS element to authorizing transactions based on time.