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?
I think there is always a solution. Does bitcoin community has developers with ability to change code and submit new feautres? If yes, that's a number one resolution. Otherwise i heard switching to POS(Proof-of-Stake) system will give node runners a reward for approving transactions.
The solution has been there right from the start, and there is no need to change anything in the code just because the cap of 21M coins will be hit one day. Mining will not stop due to no more coins to be mined. Miners will profit from transaction fees, and they are already profiting from them
right now (up to a few percentages of mining reward)...
In short, this problem is essentially nonexistent