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?
It is not impossible to mine bitcoins even after 20 years. Technological advancement are a step ahead on what's to come. Though at present it is hard to imagine but the future holds the answer to the problems that we thought today to have no solutions.
Quantum computers of high calibers in my own opinion or a more powerful computer will replace old units in mining bitcoin which is more efficient in mining bitcoins even in the future.