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
Miners will be in disasterous state at that time happens really, and as we look at the mining industry for todays they are not really profitable how much more when year 2020 come? That year could be the year of massive dumpinh of their hardwares and many bitcoin mining company will be shutdown since we all know that blocks difficulty are even more harder and difficult to find.
thats why starting from now we should minimize finding some cloudmining or even mining company to invest with so we will cannot be joined by their collapsing time.
It will take a long time for all the bitcoins will be mined and I think by that time the transaction fee will be higher, so miners will be compensated for their work.
Because no matter how you turn it, bitcoin needs miners, and miners need bitcoin.