Why is there the BitCoin limit? Because when the limit is reached we can't mine any BitCoins anymore 
And why would people then continue mining?
And why would people then continue mining?
If Bitcoin grows, then the amount of the transaction fees will be far larger in value than the mined coins.
If the fees remain small, then people will stop mining until it becomes much easier to find a block, and the miners that stay will be able to mine far more blocks than at present.