99.21875% will be mined in 20 years time. If electricity isn't very cheap by then and/or if bitcoin isn't worth hundreds of thousands of dollars, it's hard to imagine anybody wanting to mine then. How will transactions then be verified?
Why is this hard to imagine? People were mining back at the start of Bitcoin, when it had no value at all. Don't forget you don't need a lot of miners, 1 computer can be enough to confirm all transactions!
The big benefit of having a lot of miners is of course that it's harder to attack the blockchain.
Transaction fees are already a few percent of the block reward. In 20 years, especially if blocks get bigger and allow more transactions, the fees could add up to Bitcoins per block.
Actually, the transaction fees revenue is below 2% of the block reward (1.84% according to the blockchain.info
).