It is seriously hard to predict what will happen in 20 years later. Taking account of the tremendous variables, inflation, main currency at that time, how much transaction for every block, fees for every transaction, electricity cost, speed of hashes, competitors, to name a few, Anything is possible for this time span.

But in my opinion there will be much more transaction in every block and every block mined will get more transaction fees. The transaction fee will be much higher than the new bitcoin mined. Say that 1 bitcoin is equilvalent to 2500 USD not taking account of inflation, maybe some will still continue to mine, not for profitability but to contribute to the system.