Newbie here.  From what I understand the Miners are responsible for veirfying the accuracy of the transactions contained in a block and as a reward the quickest one to come up with the correct hash earns Bitcoins.  Maybe it's a stupid question but what will happen to the Bitcoin network once all 21 Million Bitcoins are in circulation and there is no longer an incentive for Miners to operate their expensive and power hungry rigs, who exit the system after selling their Bitcoins?  Who performs the transaction verification then?  Or will Bitcoin grind to a halt when that happens?