Who will be the future miners? We know that the bitcoin for mining a block is going to converge to approximately zero as the number of bitcoins will be almost constant after a certain amount of time. So, mining will no more profitable in the future and if no one is going to maintain the ledgers then how the bitcoin going to survive?
If mining isn't profitable, fewer people will mine. That means the difficulty will drop, increasing the profitability of mining. That means more people will mine. The system is, by design, self-regulating.