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?
To be honest i've think about it too, i want to know what happen with bitcoin if no miner active or all bitcoin has done mined. so we never get confirmation because no miner active?
On 20 years many thing can happen, it is too technologically speaking at the pace things are happening.