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?
At that time the block reward for miners is somewhere at 0.390625
BTC which is pretty low comparing with today's reward. 20 years from now the price should much higher to see miner still mining.
That's kind of the problem, the price
has to be higher or else the system breaks down. If the price doesn't rise, there won't be enough miners to ensure enough decentralization to make it secure. To offset, fees have to rise to keep miners in, but fees can't rise to the point where Bitcoin is too expensive to use either. Cheap, efficient, and safe are the main draws to Bitcoin at the moment, and it doesn't seem like a viable alternative if any of those elements start to break down.