Right now we're approaching about 17 million out of 21 million possible bitcoins. When we reach the max supply, what incentive would miners have to continue mining and verifying transactions? Sure they can still collect fees, but why not mine newer and more profitable coins where they have the chance to collect both fees and new coins?
What's stopping the entire blockchain from grinding to a halt, prohibiting users from conducting transactions, and thereby making bitcoin worthless?
If that happen then we will have a supply of 21 million minus the lost bitcoins, those bitcoins that their owner dies without someone taking it over. There is a speculation that the supply will be added a certain percentage. But the time that the last bitcoin will be mined is very far from now and technology by that time would be sophisticated.