There won't be any more bitcoin to be mined after the 21 million cap has been completely reached by the miners. These are not intended to work that way; rather, the software was created that way before the miners came on board. Only the 21 million bitcoins will be usable after they have all been mined, and miners will only be rewarded for validating transactions into the blockchain.

Even if the reward had significantly decreased by that point, it would still be worthwhile given how much the price of bitcoin would have increased. And given how expensive mining equipment is become, we might also see a decline in the number of miners.