We know that only 21 million Bitcoins can be mined in total. What next
Once all 21 million Bitcoins are mined, no more can be created. However, we will still need the miners to do all of the work required for completing and confirming transactions. Instead of the miners getting paid in new Bitcoins and transaction fees, they will be paid solely from transaction fees.
I can imagine that fees will be a little higher at this point but we are talking about 100 years in the future and Bitcoins will be very valuable at that point.