At current prices of $6,600 and with a block reward of 1800 coins a day (12.5 X 6 X 24) the current rewards paid out to the miners each day is $11,880,000. At some point in the future, around 2120 I think, the last bitcoin will be mined and the miners will have to live off of transaction fees alone. If that were to happen today, at todays rates, we would need to pay out the miners the roughly $11,880,000 with transaction fees alone. Something has to give. Either there will be less miners due to loss of profitability, or much higher network fees.