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?
Miners falling off would mean decreasing difficulty, so that shouldn't be a problem over a long enough time frame (i.e. difficulty matching up with the decreased hash rate). Besides, it is assumed that after almost all coins are mined, miners will earn by collecting transaction fees...
What actually happens then is hard to predict, though